System.ArgumentNullException - what mean?

[buildorbust]

What cause this error "Value cannot be null"?
Indicator starts working ok and suddenly it stops...

[NinjaTrader_Kate]

Hello buildorbust,

Thank you for your post.

Subscribing to UI events outside of the UI thread can result in the handler being null and an exception being thrown.

For example, this might trigger the above error:

this.ChartControl.KeyUp += new KeyEventHandler(KeyUpEventHandler);
this.ChartControl.KeyDown += new KeyEventHandler(KeyDownEventHandler);
this.ChartControl.MouseUp += new MouseButtonEventHandler(MouseUpEventHandler);
this.ChartControl.PreviewMouseDown += new MouseButtonEventHandler(MouseDownEventHandler);
this.ChartControl.MouseMove += new MouseEventHandler(MouseMoveEventHandler);
this.ChartPanel.MouseDoubleClick += new MouseButtonEventHandler(MouseDoubleClickEventHandler);

To fix this, the indicator would need to use a dispatcher for those UI events:

ChartControl.Dispatcher.InvokeAsync(new Action(() => {
this.ChartControl.KeyUp += new KeyEventHandler(KeyUpEventHandler);
this.ChartControl.KeyDown += new KeyEventHandler(KeyDownEventHandler);
this.ChartControl.MouseUp += new MouseButtonEventHandler(MouseUpEventHandler);
this.ChartControl.PreviewMouseDown += new MouseButtonEventHandler(MouseDownEventHandler);
this.ChartControl.MouseMove += new MouseEventHandler(MouseMoveEventHandler);
this.ChartPanel.MouseDoubleClick += new MouseButtonEventHandler(MouseDoubleClickEventHandler);
}));

If this is an indicator you've created, please make sure you're using a dispatcher for UI events.

Please let us know if we may be of further assistance to you.

[buildorbust]

mybox is ComboBox

this cause exception?
mybox.KeyDown += OnKeyDown;

correct way is:
mybox.KeyDown += new KeyEventHandler(this, OnKeyDown);
?

[NinjaTrader_Kate]

Hello buildorbust,

Thank you for your reply.

The error is because you're not wrapping that in a dispatcher. Please see the following page of our help guide for further information regarding using a dispatcher when you are adding UI elements or subscribing to UI events:

https://ninjatrader.com/support/helpGuides/nt8/multi-threading.htm

Please let us know if we may be of further assistance to you.