QmakeProject: Fix crash when deleting while pro-files are being read

Change-Id: I94c069cb806285fa23fc1a702caf620be8413e0b
Task-number: QTCREATORBUG-15954
Task-number: QTCREATORBUG-15993
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -300,6 +300,10 @@ QmakeProject::~QmakeProject()
 {
     m_codeModelFuture.cancel();
     m_asyncUpdateState = ShuttingDown;
+
+    // Make sure root node (and associated readers) are shut hown before proceeding
+    setRootProjectNode(nullptr);
+
     projectManager()->unregisterProject(this);
     delete m_projectFiles;
     m_cancelEvaluate = true;