Modified WinForms backend to prevent error when window content is not set.

[proneon267]

Modified WinForms backend to prevent error when window content is not set.

Prevents non meaningful error messages like the following from showing up:

...
  File "...\winforms\src\toga_winforms\window.py", line 124, in show
    self.interface.content.refresh()
AttributeError: 'NoneType' object has no attribute 'refresh'

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Looks reasonable.

As an aside - while these bug fixes to Windows are great, they should all be picked up as part of #2058 being completed, and that PR includes automated tests of behavior.