The value of insertIMETextCommandContext.IMEStartPosition is null in the methord  Telerik.Windows.Documents.UI.CaretTextInputHandler.HandleTextInsertedWithIme.

This happens when typing with IME in a position without editing permissions, such as a protected document or a content locked ContentControl.

I tried turning on Use previous version of Microsoft IME, the result was the same.

I've also tested this on my other 2 machines with Windows 7 and Windows 10 installed,  and the results were the same.

My current workaround is to modify the code of the method Telerik.Windows.Documents.UI.CaretTextInputHandler.HandleTextInsertedWithIme，add some null value checks.

 

System.NullReferenceException
  HResult=0x80004003
  Message=未将对象引用设置到对象的实例。
  Source=Telerik.Windows.Documents
  StackTrace:
   在 Telerik.Windows.Documents.DocumentPosition..ctor(DocumentPosition documentPosition, Boolean trackDocumentChangeEvents) 在 Telerik.Windows.Documents\DocumentPosition.cs 中: 第 740 行
   在 Telerik.Windows.Documents.UI.CaretTextInputHandler.HandleTextInsertedWithIme(RadDocument document, String text, TextInsertedEventArgs e) 在 Telerik.Windows.Documents.UI\CaretTextInputHandler.cs 中: 第 193 行
   在 Telerik.Windows.Documents.UI.Caret.<>c__DisplayClass80_0.<OnTextInserted>b__0() 在 Telerik.Windows.Documents.UI\Caret.cs 中: 第 482 行
   在 System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   在 System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   在 System.Windows.Threading.DispatcherOperation.InvokeImpl()
   在 MS.Internal.CulturePreservingExecutionContext.CallbackWrapper(Object obj)
   在 System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   在 MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
   在 System.Windows.Threading.DispatcherOperation.Invoke()
   在 System.Windows.Threading.Dispatcher.ProcessQueue()
   在 System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   在 MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   在 MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   在 System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   在 System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   在 System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   在 MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   在 MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   在 System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   在 System.Windows.Application.RunDispatcher(Object ignore)
   在 System.Windows.Application.RunInternal(Window window)
   在 TelerikEditorDemo.App.Main()

I think it is because your caret is not inside the content control. Use the arrow keys to move the caret inside the ContentControl and try again.