A REPL example to reproduce the issue is:
https://blazorrepl.telerik.com/mIuTdbuT08QWQoY319

The error is:

Adding a new appointment in the Gantt throws an System.ArgumentNullException. The issue can be reproduced only in WASM application.

Steps To Reproduce:

1. Run REPL example
2. Double-click on some appointment in the timeline
3. Go to Predecessors tab
4. Try to Add new item

I have a page with a Gantt chart and a Button. The button navigates to another page in the app (NavigationManager.NavigateTo()). When I click the button an exception is thrown:

Exception trace: Error: Microsoft.JSInterop.JSException: An exception occurred executing JS interop: The JSON value could not be converted to System.Double. Path: $ | LineNumber: 0 | BytePositionInLine: 4.. See InnerException for more details. ---> System.Text.Json.JsonException: The JSON value could not be converted to System.Double. Path: $ | LineNumber: 0 | BytePositionInLine: 4.
 ---> System.InvalidOperationException: Cannot get the value of a token type 'Null' as a number.

How to get only the year and date in the Gantt Chart Tooltip template? The DataAttribute is not a valid date string.

<TooltipTemplate>
     <h4>@(((TooltipTemplateContext)context).Title)</h4>
     <h5>Start: @(((TooltipTemplateContext)context).DataAttributes["start"])</h5>
     <h5>End: @(((TooltipTemplateContext)context).DataAttributes["end"])</h5>

</TooltipTemplate>

Zooming levels would be great

GanttCommandButton Command="ZoomIn|ZoomOut|ZoomToFit"...>

The current Views (Day/Week/Month/Year) are nice, but custom zooming is needed sometimes.

By default, all nested tasks are displayed expanded.
The state of the root task is always the most important, so it makes more sense to display tasks minimized by default.
The user can always expand the required task for details.
The solution is to support the "IsExpanded" property for the task model.
When a property changes, the task changes its state.
For the component, also add the "ExpandAll" and "CollapseAll" methods that affect all tasks in the list.

When I describe a EditorTemplate on a GanttColumn, I expect it to be shown on entering the edit-mode:

<GanttColumn Field="@nameof(ActivityContainer.TaskName)" Title="@_["_name"]" Expandable="true" Resizable="true" >
    <EditorTemplate>
        <strong>I expect this to see</strong>
    </EditorTemplate>
</GanttColumn>

Expected: Just the string "I expect this to see"

What I actually get:

The default edit box (feels like the EditorTemplate doesn't apply at all?)

Docs (https://docs.telerik.com/blazor-ui/api/Telerik.Blazor.Components.GanttColumn#collapsible-Telerik_Blazor_Components_GanttColumn_EditorTemplate) say, that the GanttColumn has a EditorTempalte.

---

Side effect:

<GanttColumn Field="@nameof(ActivityContainer.TaskName)" Title="@_["_name"]" Expandable="true" Resizable="true" >
    <EditorTemplate>
        <strong>@((context as ActivityContainer)?.TaskName)I expect this to see</strong>
    </EditorTemplate>
</GanttColumn>

error CS0103: The name 'context' does not exist in the current context

Context should be there.

It is very strange that while the Gantt component is similar to the gorgeous Grid component, support for row and header templates has not been released.
We await implementation
<GanttColumn>

  <HeaderTemplate Context = @ myContext>

  </HeaderTemplate>

  <Template Context = @ myContext>

  </Template>

</GanttColumn>

**Admin Edit**

Will be released in 3.1.0.

**Admin Edit**

The new Gantt feature turned out really great. It would be nice if the relationships such as predecessor and successor were added, as well as different relationship types (e.g. start-end or start-start) with corresponding offsets.

Likewise, the representation of the critical path and a baseline.