Nonexistent QT_STYLE_OVERRIDE style prevents main window from being shown

With Qt6, when the QT_STYLE_OVERRIDE environment variable references an "invalid" (nonexistent) style, it prevents the main window from being shown, and the following generic/non-descriptive message is printed to the terminal:

qrc:/src/MainApplicationWindow.qml: module "adwaita" is not installed

I believe this is a regression from Qt5: with Qt5, the main window would open and be displayed just fine, and a helpful warning similar to the following would be printed (which is no longer printed with Qt6):

QApplication: invalid style override 'adwaita' passed, ignoring it.
    Available styles: Windows, Fusion

I stumbled upon this when testing a Qt application on Zorin OS 16, where they hardcode QT_STYLE_OVERRIDE=adwaita but the custom version of Qt I was testing with did not have the adwaita style built into it, and as a result the main window would not be shown.