Remove unnecessary null pointer checks

[elfring]

An extra null pointer check is not needed in functions like the following.

• AppearancePage
• Qt6CTPlatformTheme
• TroubleshootingPage::on_removeAppButton_clicked

[trialuser02]

If possible, please create merge request for this.

[elfring]

:thought_balloon: Would you become interested to use a development tool like “clang-tidy” for corresponding source code adjustments?

[elfring]

Thanks for your small source code improvements.

[elfring]

:thought_balloon: Another update candidate was left over.