Plugins: Fix nullptr access

Some plugins access their private "d" member in their destructor.
If Plugin initialization failed, these might not have been created.

This would lead to a crash.

Change-Id: Ifd916daf90ebac9a8933dd5489ec1ac0a38254a0
Reviewed-by: hjk <hjk@qt.io>

src/plugins/qmlprojectmanager/qmlprojectplugin.cpp

@@ -100,6 +100,8 @@ public:
 
 QmlProjectPlugin::~QmlProjectPlugin()
 {
+    QTC_ASSERT(d, return);
+
     if (d->lastMessageBox)
         d->lastMessageBox->deleteLater();
     if (d->landingPage)