Crashes in qwindows.dll (was QWindow::setVisible crash)

• We're experiencing this crash at least since 5.2.1 and it also happens with 5.3.1
• It doesn't happen in debug and release on our machines
• What we just have is numerious crash reports with .dmp files

After analyzing .dmp files we came to the following conclusion:

QWindow::setVisible crashes on the following line with trying to read from 0x00000000

It's exactly this source line:
d->platformWindow->setVisible(visible);

It means that for some unknown reason d->platformWindow is 0x00000000. Users report that the app crashes when restoring program main window from Windows tray by calling QWidget::showNormal() from QMainWindow inherited class

IMPORTANT We keep receiving crash reports with the following scenario: user doesn't see app tray icon, tries to open another instance, the first instance calls showNormal from QMainWindow - CRASH ("crash_callstack_more.png")

The thing is it's NOT crashing on startup, it works fine. After some time it crashes after it's running in background in system tray. After that the customer can use the app for weeks until the next crash. Maybe it somehow depends on software installed because the crash never happened on our PCs.

Screenshots illustrating the problem are attached.

UPDATE: It also crashes in 5.4.1 but with a different call stack. Once we reproduced the crash: suddenly noticed that no tray icon is visible, but the app process was active, we ran the second instance of the app, then the first instance crashed (when trying to display main window). See callstack_5.4.1.png
and YES, we keep getting crash reports from users