Break a infinite loop.

Removing the last project did set startupproject to zero and unloaded
the project. Thus the check that the project for the current item and
the startup project are both null and the same.

src/plugins/projectexplorer/projectwindow.cpp

@@ -310,12 +310,14 @@ void ProjectWindow::handleCurrentItemChanged(QTreeWidgetItem *current)
 
 void ProjectWindow::handleItem(QTreeWidgetItem *item, int column)
 {
-
     if (!item || column != 1) // startup project
         return;
 
     const QString path = item->data(2, Qt::UserRole).toString();
     Project *project = findProject(path);
+    // Project no longer exists
+    if (!project)
+        return;
      if (!(item->checkState(1) == Qt::Checked)) { // is now unchecked
          if (m_session->startupProject() == project) {
              item->setCheckState(1, Qt::Checked); // uncheck not supported