Fix toolmanager's destroy subplots in tk

[ImportanceOfBeingErnest]

PR Summary

Fixes issue raised in https://stackoverflow.com/questions/56220821/python-matplotlib-removing-configure-subplots-tool-from-toolbar-in-matplotlib

Using the Tk backend in conjunction with the 'toolmanager' toolbar, the following line of code

 fig.canvas.manager.toolmanager.remove_tool('subplots')

would fail with an error

AttributeError: 'NoneType' object has no attribute 'destroy'

when being called before any window is instantiated.

This PR checks if self.window is None and only attempts to destroy existing windows.

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[timhoffm]

Thanks.