Guard empty printer preview loading for issues #6 and #25

[ShredderAlex]

Description

This PR guards printer preview loading when printertype or printermodel is empty, preventing the crash/incorrect loading state described in issue #6 and the slow/failed filament selection behavior in issue #25.

Screenshots/Recordings/Graphs

None.

Tests

Not run; this is a targeted logic fix.

[bitbarista]

Thanks for the PR! Just a note: the core issue in #6 and #25 (on Linux/Wayland) is actually a missing ShouldDismissOnTopWindowDeactivate() guard in PopupWindow.cpp — without it, the filament selector popup dismisses itself the instant it opens because the main window briefly loses activation when the popup takes focus.

The fix is a two-line backport from SoftFever's commit c0ae2bd (details in #6). GDK_BACKEND=x11 also works around it at launch time.

This PR may fix a separate crash for users with an empty printer_type, but it won't resolve the Wayland popup dismissal behaviour described in those issues.

[ShredderAlex]

Thanks for the feedback, bitbarista. The suggested fix has now been applied.

Best regards,
Alexander Schröder