Windows QPA: Potential crash fix for Wine

Add a check for theme instance.

Pick-to: 5.15
Fixes: QTBUG-85734
Change-Id: I10aced73379cd5e0b49d5cee08ceead060397a54
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/windows/qwindowsscreen.cpp

@@ -569,8 +569,10 @@ bool QWindowsScreenManager::handleScreenChanges()
                 removeScreen(i);
         }     // for existing screens
     }     // not lock screen
-    if (primaryScreenChanged)
-        QWindowsTheme::instance()->refreshFonts();
+    if (primaryScreenChanged) {
+        if (auto theme = QWindowsTheme::instance()) // QTBUG-85734/Wine
+            theme->refreshFonts();
+    }
     QHighDpiScaling::updateHighDpiScaling();
     return true;
 }