ProjectExplorer: Create initial project tree

Create an initial project tree with a ProjectNode and a FileNode for
the project file itself.

Fix the Projects to not implement their own tree before they have
better data.

Change-Id: I147ccd5603d22d1d60880a97f30fd8c271eac88c
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/session.cpp

@@ -200,6 +200,8 @@ void SessionManager::updateProjectTree(Project *pro)
             break;
         }
     }
+    if (!currentPair)
+        return;
 
     if (!currentPair)
         return; // Project was already de-registered and is shutting down