TDT 3.3.1 (beta) crashes on opening second instance

[shulist]

Expected Behaviour: A second instance of TDT should open when the icon is selected.
Actual Behaviour: The app freezes and then crashes with the error message

An unexpected error ocurred Object reference not set to an instance of an object. Please see Help -> Show Error Log for more details

Error Log:

[2019-02-11 16:47:34] An unexpected error occurred
System.NullReferenceException: Object reference not set to an instance of an object.
   at Client.MainWindow.OnClosed(EventArgs e)
   at System.Windows.Window.WmDestroy()
   at System.Windows.Window.WindowFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at System.Windows.Interop.HwndSource.PublicHooksFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

[caseydiers]

This appears to have been previously fixed as I use multiple instances regularly and have never run into this issue.