CppEditor: Fix potential null pointer dereference

Detected by Address Sanitizer.

Change-Id: Ieab9c201e1fe6a7cf5c32747b2ce3d4562f4e598
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/cppeditor/cppeditorplugin.cpp

@@ -291,7 +291,9 @@ ExtensionSystem::IPlugin::ShutdownFlag CppEditorPlugin::aboutToShutdown()
 
 static CppEditorWidget *currentCppEditorWidget()
 {
-    return qobject_cast<CppEditorWidget*>(EditorManager::currentEditor()->widget());
+    if (IEditor *currentEditor = EditorManager::currentEditor())
+        return qobject_cast<CppEditorWidget*>(currentEditor->widget());
+    return 0;
 }
 
 void CppEditorPlugin::switchDeclarationDefinition()