QNX: Don't crash if we unplug the primary display.

The QPA plugin assumes in several places that we have at least
one QScreen.

Even if patching the plugin to support 0 screens, Qt itself crashes
when dereferencing a null paint device while synching the backing
store.

Change-Id: I2ac504a447aff811d6c07ab857340a3433557cdc
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

src/plugins/platforms/qnx/qqnxscreeneventhandler.cpp

@@ -42,6 +42,7 @@
 #include "qqnxscreeneventhandler.h"
 #include "qqnxintegration.h"
 #include "qqnxkeytranslator.h"
+#include "qqnxscreen.h"
 
 #include <QDebug>
 #include <QGuiApplication>
@@ -487,11 +488,20 @@ void QQnxScreenEventHandler::handleDisplayEvent(screen_event_t event)
             m_qnxIntegration->createDisplay(nativeDisplay, false /* not primary, we assume */);
         }
     } else if (!isAttached) {
+        // We never remove the primary display, the qpa plugin doesn't support that and it crashes.
+        // To support it, this would be needed:
+        // - Adjust all qnx qpa code which uses screens
+        // - Make QWidgetBackingStore not dereference a null paint device
+        // - Create platform resources ( QQnxWindow ) for all QWindow because they would be deleted
+        //   when you delete the screen
+
+        if (!screen->isPrimaryScreen()) {
             // libscreen display is deactivated, let's remove the QQnxScreen / QScreen
             qScreenEventDebug() << "removing display";
             m_qnxIntegration->removeDisplay(screen);
         }
     }
+}
 
 #include "moc_qqnxscreeneventhandler.cpp"