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/projectexplorer/projectexplorer.cpp

@@ -788,6 +788,8 @@ ProjectExplorerPlugin::ProjectExplorerPlugin()
 
 ProjectExplorerPlugin::~ProjectExplorerPlugin()
 {
+    QTC_ASSERT(dd, return);
+
     delete dd->m_proWindow; // Needs access to the kit manager.
     JsonWizardFactory::destroyAllFactories();
 

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)

src/plugins/vcsbase/vcsplugin.cpp

@@ -24,6 +24,7 @@
 
 #include <utils/futuresynchronizer.h>
 #include <utils/macroexpander.h>
+#include <utils/qtcassert.h>
 
 #include <QDebug>
 
@@ -51,6 +52,7 @@ VcsPlugin::VcsPlugin()
 
 VcsPlugin::~VcsPlugin()
 {
+    QTC_ASSERT(d, return);
     d->m_synchronizer.waitForFinished();
     VcsOutputWindow::destroy();
     m_instance = nullptr;