The issue only reproduces on Windows 10( 1806). Check the attached video showing the result: 

Workaround: Handle the CommandExecuting event this way
private void RadRichTextEditor1_CommandExecuting(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
{
    if (e.Command is ChangeFontFamilyCommand && e.CommandParameter.ToString() == "")
    {
        e.Cancel = true; 
    }
}

To reproduce: run the Demo application >> RichTextEditor >> First Look example and follow the steps illustrated in the attached gif file. 

1. First add a footnote in the text
2. Than select a part of the footnote text or all text and use the context menu to cut the text
3. Then,  paste the text in the document, it doesn't matter were.
4. Next step is to remove the footnote reference.
5.Then, scroll through the document and press the right mouse button to open the context menu.

The object release should have been called and the context menu is disposed. If so the error occurs.

Workaround:

this.radRichTextEditor1.MouseClick += radRichTextEditor1_MouseClick;

        private void radRichTextEditor1_MouseClick(object sender, MouseEventArgs e)
        {
            if (e.Button == System.Windows.Forms.MouseButtons.Right)
            {
                if (this.radRichTextEditor1.RichTextBoxElement.ContextMenu != null)
                {
                    FieldInfo fi = typeof(Telerik.WinControls.RichTextEditor.UI.ContextMenu).GetField("radDropDownMenu", BindingFlags.Instance | BindingFlags.NonPublic);
                    RadDropDownMenu dropdown = fi.GetValue(this.radRichTextEditor1.RichTextBoxElement.ContextMenu) as RadDropDownMenu;
                    if (dropdown.IsDisposed)
                    {
                        this.radRichTextEditor1.RichTextBoxElement.ContextMenu = new Telerik.WinControls.RichTextEditor.UI.ContextMenu();
                    }
                }
            }
        }

Add a property that can control whether a table row is allowed to break across pages or not.

To reproduce: please refer to the attached sample project and follow the steps in the gif file. 

Workaround: 

        private void radRichTextEditor1_CommandExecuting(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
        {
            if (e.Command  is ChangeIntegratedWatermarkTypeCommand && this.radRichTextEditor1.LayoutMode!= Telerik.WinForms.Documents.Model.DocumentLayoutMode.Paged)
            {
                e.Cancel = true; 
            }
        }

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at Telerik.WinForms.Documents.UI.UIProviders.WatermarkUIProvider.Arrange(UILayerUpdateContext uILayerUpdateContext)
   at Telerik.WinForms.Documents.UI.Layers.ProviderUILayerBase.ArrangeChildren()
   at Telerik.WinForms.Documents.UI.DocumentPrintPresenter.ArrnageUILayers()
   at Telerik.WinForms.Documents.UI.DocumentPrintPresenter.MeasureOverride(SizeF availableSize)
   at Telerik.WinControls.RadElement.MeasureCore(SizeF availableSize)
   at Telerik.WinControls.RadElement.Measure(SizeF availableSize)
   at Telerik.WinControls.RadElement.MeasureChildren(SizeF availableSize)
   at Telerik.WinControls.RichTextEditor.UI.FrameworkElement.MeasureOverride(SizeF availableSize)
   at Telerik.WinControls.RadElement.MeasureCore(SizeF availableSize)
   at Telerik.WinControls.RadElement.Measure(SizeF availableSize)
   at Telerik.WinControls.Layouts.ContextLayoutManager.UpdateLayout()
   at Telerik.WinForms.Documents.UI.DocumentPrintPresenter.Telerik.WinForms.RichTextEditor.IDocumentEditorPresenter.UpdateLayout()
   at Telerik.WinForms.RichTextEditor.RadRichTextBox.<>c__DisplayClass3.<UpdateEditorLayout>b__2()
   at Telerik.WinForms.RichTextEditor.RadRichTextBox.UpdateEditorLayout(Boolean focusCarret, Boolean updateCaretSize, Boolean async)
   at Telerik.WinForms.RichTextEditor.RadRichTextBox.set_ActiveEditorPresenter(IDocumentEditorPresenter value)
   at Telerik.WinForms.RichTextEditor.RadRichTextBox.Telerik.WinControls.UI.IPrintable.BeginPrint(RadPrintDocument sender, PrintEventArgs args)
   at Telerik.WinControls.UI.RadPrintDocument.OnBeginPrint(PrintEventArgs e)
   at System.Drawing.Printing.PrintDocument._OnBeginPrint(PrintEventArgs e)
   at System.Drawing.Printing.PrintController.Print(PrintDocument document)
   at System.Drawing.Printing.PrintDocument.Print()
   at System.Windows.Forms.PrintPreviewControl.ComputePreview()
   at System.Windows.Forms.PrintPreviewControl.CalculatePageInfo()
   at System.Windows.Forms.Control.InvokeMarshaledCallbackDo(ThreadMethodEntry tme)
   at System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Forms.Control.InvokeMarshaledCallback(ThreadMethodEntry tme)
   at System.Windows.Forms.Control.InvokeMarshaledCallbacks()


To reproduce: please refer to the attached gi file demonstrating how to replicate the problem with the Demo application.

Workaround:

        private void radRichTextEditor1_CommandExecuting(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
        {
            if (e.Command  is ChangeIntegratedWatermarkTypeCommand && this.radRichTextEditor1.LayoutMode!= Telerik.WinForms.Documents.Model.DocumentLayoutMode.Paged)
            {
                e.Cancel = true; 
            }
        }

Workaround: If possible set the layout mode to FlowNoWrap
this.radRichTextEditor1.Document.LayoutMode = Telerik.WinForms.Documents.Model.DocumentLayoutMode.FlowNoWrap;

Currently unknown/unsupported elements are skipped, which potentially loses text content. Consider adding setting in HtmlImportOptions allowing the phrasing content (text) of these elements to be imported. 

Sample HTML:

<html>
<body>
<test-test>test-test</test-test>
<body/>
</html>


Browsers visualize it as "test-test", while RadRichTextBox doesn't import anything.

MS Word imports such elements as inline elements (just like spans).

To reproduce: run the project and maximize the form. When you enter some text you will notice that the text is cut off.

Workaround: use paged layout.

<samp> HTML element could be imported as span with specific formatting. Currently it's just ommited.

Workaround:

        public TextEditorRadForm()
        {
            InitializeComponent();
            ((RadForm)this.radRichTextEditor1.RichTextBoxElement.InsertCaptionDialog).Controls["radRichTextEditorLabel"].Text = "";
        }

To reproduce: add a RadRichTextEditor with ribbon UI. Set the RichTextEditorRibbonBar's theme to Fluent at design time. When you run the application you will notice that the theme is not properly applied to the backstage view.

Workaround: ThemeResolutionService.ApplicationThemeName = "Fluent";

To reproduce:
- Drop RadRichTextEditor  to a form.
- Set the theme to FluentDark. 

Workaround:
radRichTextEditor1.ChangeTextForeColor(Color.White);

or

public void SetBackColor()
{
    this.radRichTextEditor1.RichTextBoxElement.BackColor = Color.White;
    Theme theme = ThemeRepository.FindTheme("TelerikMetroBlue");

    foreach (var styleGroup in theme.StyleGroups)
    {
        foreach (PropertySettingGroup settingGroup in styleGroup.PropertySettingGroups)
        {
            if (settingGroup.Selector.Value == "Telerik.WinControls.RichTextEditor.UI.Page")
            {
                foreach (PropertySetting property in settingGroup.PropertySettings)
                {
                    if (property.Name == "BackColor")
                    {
                        property.Value = System.Drawing.Color.White;
                    }
                }
            }
            if (settingGroup.Selector.Value == "Telerik.WinControls.RichTextEditor.UI.HeaderFooterContainer")
            {
                foreach (PropertySetting property in settingGroup.PropertySettings)
                {
                    if (property.Name == "OverlayColor")
                    {
                        property.Value = System.Drawing.Color.White;
                    }
                }
            }
        }
    }

}

Applying negative LeftIndent or FirstLineIndent of a paragraph in web layout mode will visualize the paragraph to the left of RadRichTextBox boundaries.

Use attached to reproduce. 

When there is a hyperlink enclosed in another annotation (e.g comment, bookmark, read-only range), and the user moves the caret position just before or just after the hyperlink, and starts typing there, the text is inserted with the style of the hyperlink (by default, blue with blue underline). Instead, the text should be inserted without such style.

To reproduce: run the project, load the document available in the project's folder and start clicking the words. You will notice that the cursor is shifted on the right. 

Workaround: don't enable the RightToLeft property.

Provide functionality allowing a document to be thumbnailed. The customers should be able to export a document to an image.

Workaround: Use attached project.