ProjectWindow: Initialize vanishedTargetsItem

We check for its existence in rebuildVanishedTargets(),
so this was a potential crash.

Change-Id: I5be2f4bad54cc9b1ce16e2f0ab7e63378379803d
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/projectwindow.cpp

@@ -594,7 +594,7 @@ private:
     int m_currentChildIndex = 0; // Start with Build & Run.
     Project *m_project = nullptr;
     TargetGroupItem *m_targetsItem = nullptr;
-    VanishedTargetsGroupItem *m_vanishedTargetsItem;
+    VanishedTargetsGroupItem *m_vanishedTargetsItem = nullptr;
     MiscSettingsGroupItem *m_miscItem = nullptr;
     const std::function<void ()> m_changeListener;
 };