Forms9 - Windows and Canvaces ?

Hi, All.

I have a problem:
In a Form I've created main_window and main_canvas (content) for main_window, then segments_window and segments_canvas (content) for segments_window.
When I compile this Form I have an Error:

FRM-30029: Cannot find window SEGMENTS_WINDOW.
View Canvas SEGMENTS_CANVAS
Form: FORM_MODULE
FRM-30085: Unable to adjust form for output.

Is it my misunderstanding or is it Forms's bug?
/------------- Cut from Forms's Help ---------------
Note: You must assign at least one content canvas to every window you create. To assign a canvas to a window, set the canvas' Window property to the appropriate window.
/------------- End Cut ---------------

Thanks to All answered. I've found out about my problem.