Add a possibility to set the auto-expand delay on drag-drop in RadTreeView.

After executing the following code and checking the checkbox of the node, a NullReferenceException is thrown:

this.radTreeView1.BeginUpdate(); 
this.radTreeView1.Nodes.Clear();
radTreeView1.Nodes.Add(new RadTreeNode("name")); 
this.radTreeView1.EndUpdate();

1. Create a new project with RadTreeView
2. Run the project
3. Click inside RadTreeView
4. Press the up/down arrows several times
5. An exception will occur

1. Create a new project and add RadTreeView
2. Add Add and Clear buttons
3. On Add button click - add nodes
4. On Clear button click - call the Clear method of the Nodes collection
5. Run the project and click the Add/Clear methods several times

FIX. RadTreeView - the GradientStyle property of RadTreeNode is overriden by the theme, even though it is set locally
The first part of the issue, regarding RadTreeNode.BackColor is addressed.
There is no issue with the persistance of GradientStyle property. To persist the GradientStyle property you should set its value explicitly. Here is an example:
void radTreeView1_NodeFormatting(object sender, TreeNodeFormattingEventArgs e)
{
if (radTreeView1.SelectedNode != null)
{
if (e.Node == radTreeView1.SelectedNode)
{
e.NodeElement.BackColor = Color.Yellow;
e.NodeElement.BackColor2 = Color.White;
e.NodeElement.GradientStyle = Telerik.WinControls.GradientStyles.Linear;
e.NodeElement.DrawFill = true;
e.NodeElement.NumberOfColors = 2;
return;
}
}
e.NodeElement.ResetValue(LightVisualElement.BackColorProperty, Telerik.WinControls.ValueResetFlags.Local);
e.NodeElement.ResetValue(LightVisualElement.BackColor2Property, Telerik.WinControls.ValueResetFlags.Local);
e.NodeElement.ResetValue(LightVisualElement.GradientStyleProperty, Telerik.WinControls.ValueResetFlags.Local);
e.NodeElement.ResetValue(LightVisualElement.DrawFillProperty, Telerik.WinControls.ValueResetFlags.Local);
e.NodeElement.ResetValue(LightVisualElement.NumberOfColorsProperty, Telerik.WinControls.ValueResetFlags.Local);
}

1. Create a new project with RadTreeView
2. Handle the NodeCheckedChanged event
3. Call the ExpandAll method for the checked node when handling this event
4. Run the project and check the first node

Workaround: 
private void radTreeView1_NodeFormatting(object sender, TreeNodeFormattingEventArgs e)
{
    e.NodeElement.ContentElement.TextAlignment = ContentAlignment.MiddleCenter;
}

Improve the TreeViewDragDropService extensibility by allowing replacing the default remove behavior with copy behavior.

ValueValidating and ValidationError are fired twice when message box is shown in the event handler of ValidationError event.

1. Create a new project containing a RadTreeView
2. Add some nodes
3. On a Button.Click event call the Collapse method for the selected node
4. Run the application and click the button

There should be a convenient way to hide the expander icon and to show lines when using full lazy mode.

When i have the root nodes loaded, but there are more nodes than fit in the visible area of the treeview, the scrollbar is not automatically shown. I've tried setting the AutoScroll option to true, but that doesn't help.

When the ShowLines property is set to true, the lines that link nodes in RadTreeView are not rendered appropriately for some levels.
Comment: When using full lazy mode RadGridView displays expander icon for all nodes. When it is hidden there is a whitespace. You can show tree lines only with code.

Buttons are not visible when using custom DPI settings in RadTreeView property builder.

There is no way to reset the style of node to its default one by using its Style property.

While a node is dragged and right mouse button is clicked, the dragged node is replaced with the selected node.

SelectedNodeChanged event not owrk properly during remove operation of SelectedNode

FIX. RadTreeView - Collapse method of RadTreeNode throws an exception if uised afrer children are added

FIX RadTreeView - the CheckedNodes property should return the number of the logical checked nodes, not the visual once

If the ShowLines property is set to true and a node's expander is hidded, a link line should appear instead.
Comment: When using full lazy mode RadGridView displays expander icon for all nodes. When it is hidden there is a whitespace. You can show tree lines only with code.