ProjectExplorer: Treat empty root project nodes better

Do not set up empty root project nodes and fall back to the default
project tree instead. A project can not ever be empty: It should at
least include the project file itself so that the user can fix
breakages when project parsing fails.

Change-Id: I692e299f56727305120777cdc532607d5b0be99e
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/projectexplorer/project.cpp

@@ -461,6 +461,14 @@ void Project::setRootProjectNode(ProjectNode *root)
     if (d->m_rootProjectNode == root)
         return;
 
+    if (root && root->nodes().isEmpty()) {
+        // Something went wrong with parsing: At least the project file needs to be
+        // shown so that the user can fix the breakage.
+        // Do not leak root and use default project tree in this case.
+        delete root;
+        root = nullptr;
+    }
+
     ProjectTree::applyTreeManager(root);
 
     ProjectNode *oldNode = d->m_rootProjectNode;