packagekit: Prevent removing SystemUpgrade, don't crash

If the user was gambling his luck by trying to uninstall the "System
Upgrade" entry we create for offline upgrade, now they'll get an error
message rather than a crash.

libdiscover/backends/PackageKitBackend/PackageKitBackend.cpp

@@ -655,6 +655,10 @@ Transaction* PackageKitBackend::installApplication(AbstractResource* app)
 Transaction* PackageKitBackend::removeApplication(AbstractResource* app)
 {
     Q_ASSERT(!isFetching());
+    if (!qobject_cast<PackageKitResource*>(app)) {
+        Q_EMIT passiveMessage(i18n("Cannot remove '%1'", app->name()));
+        return nullptr;
+    }
     return new PKTransaction({app}, Transaction::RemoveRole);
 }