guard against invalid applets

an applet can be present but being one that failed to load
therefore with an invalid kplugininfo, which can cause asserts around

BUG:364281

src/plasmaquick/configview.cpp

@@ -85,6 +85,10 @@ void ConfigViewPrivate::init()
         qWarning() << "Null applet passed to constructor";
         return;
     }
+    if (!applet.data()->pluginInfo().isValid()) {
+        qWarning() << "Invalid applet passed to constructor";
+        return;
+    }
 
     applet.data()->setUserConfiguring(true);