PluginView: Fix crash

Without a running QC we still need to initialize the
plugin manager properly.

Change-Id: Ia2a83080f68de9f88d214f5afbcb6a07f31daea9
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

tests/manual/pluginview/plugindialog.cpp

@@ -129,6 +129,8 @@ int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);
     ExtensionSystem::PluginManager manager;
+    manager.setSettings(new QtcSettings);
+
     manager.setPluginIID(QLatin1String("plugin"));
     setCreatorTheme(new Theme("default", &app));
     QObject::connect(&app, &QCoreApplication::aboutToQuit,