Difficult to explain - I had been doing a lot of things, and not sure which of them triggered it :-(
However, this morning I received it again, just after I started MusicBee. I had opened an Auto-Playlist, and had right-clicked on it to edit the parameters when the error message appeared.
MusicBee v3.4.7805.33439D (Win10.0), 29 Sep 2021 8꞉33:
System.NullReferenceException: Object reference not set to an instance of an object.
at #=zziapYCHkKIL0B5EplaYmmhGl87_u.#=zlhwt0Rjy9D6H()
at #=zziapYCHkKIL0B5EplaYmmhGl87_u.#=zqIxP4gNj$RPc.#=zFHXiStHtS0CzX87S0kKfoUw=(Object #=zrGRMGbM=, EventArgs #=zJqXIhV8=)
at System.EventHandler.Invoke(Object sender, EventArgs e)
at System.Windows.Forms.ToolStripDropDownItem.OnDropDownShow(EventArgs e)
at System.Windows.Forms.ToolStripMenuItem.OnDropDownShow(EventArgs e)
at System.Windows.Forms.ToolStripDropDownItem.ShowDropDownInternal()
at System.Windows.Forms.ToolStripDropDownItem.ShowDropDown(Boolean mousePush)
at System.Windows.Forms.ToolStripMenuItem.OnMenuAutoExpand()
at System.Windows.Forms.MenuTimer.OnTick(Object sender, EventArgs e)
at System.Windows.Forms.Timer.OnTick(EventArgs e)
at System.Windows.Forms.Timer.TimerNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)