ProjectExplorer: Delete all the widgets when deleting the project tree

Leaving them around is dangerous as they rely on the tree to be valid.
When unloading the plugin the ProjectTree instance is deleted.

Change-Id: Iea6524b0c57c469045cb35f3cd2f376e579b8a57
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/projecttree.cpp

@@ -32,6 +32,7 @@
 #include "nodesvisitor.h"
 
 #include <utils/algorithm.h>
+#include <utils/qtcassert.h>
 #include <coreplugin/icore.h>
 #include <coreplugin/editormanager/ieditor.h>
 #include <coreplugin/editormanager/editormanager.h>
@@ -78,11 +79,19 @@ ProjectTree::ProjectTree(QObject *parent) : QObject(parent)
             this, &ProjectTree::sessionChanged);
 }
 
+ProjectTree::~ProjectTree()
+{
+    QTC_ASSERT(s_instance == this, return);
+    s_instance = nullptr;
+}
+
 void ProjectTree::aboutToShutDown()
 {
     disconnect(qApp, &QApplication::focusChanged,
                s_instance, &ProjectTree::focusChanged);
     s_instance->update(0, 0);
+    qDeleteAll(s_instance->m_projectTreeWidgets);
+    QTC_CHECK(s_instance->m_projectTreeWidgets.isEmpty());
 }
 
 ProjectTree *ProjectTree::instance()

src/plugins/projectexplorer/projecttree.h

@@ -44,6 +44,7 @@ class PROJECTEXPLORER_EXPORT ProjectTree : public QObject
     Q_OBJECT
 public:
     explicit ProjectTree(QObject *parent = nullptr);
+    ~ProjectTree();
 
     static ProjectTree *instance();