Fix exception on window style change

[webfiltered]

Guards against exception. Repro:

1. Change from default window style to custom via setWindowStyle
2. Send any window controls theme update (change button size / colour)
3. Throws

Solution

AppWindow now checks the settings it was created with, rather than the current settings.

Cause

Original frontend impl. was incomplete (settings), so the flaw went unnoticed.

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