Expected Behavior

The documentation lists add(*children) as a method of the Canvas widget. I expect to be able to add other child widgets to the Canvas widget.

Current Behavior

I am attempting to add an ImageView as a child of the Canvas widget, but receive "ValueError: Cannot add children"

Steps to reproduce

1. Create canvas widget
   canvas = toga.Canvas()

2. Create another widget (ie ImageView)
   image = toga.ImageView()

3. Attempt to add ImageView as child of Canvas
   canvas.add(image)

Your Environment

• Python Version (list the specific version number)
  3.7.7

• Operating System and Version (select from the following and list the specific version number; if your OS is not listed, list that as well)

  • Windows - version: 10 build 1903

• Toga Version (list the specific version number or git hash)
  0.3.0.dev21

• Toga Target (the type of app you are trying to generate)

  • android
  • cocoa
  • django
  • gtk
  • iOS
  • tvOS
  • watchOS
  • winforms
  • win32
  • Other (please specify)