Valgrind: Fix a crash on shutdown

Ignore engineFinished() on shutdown while callgrind was still running.
At this stage the m_errorView, which was added to the perspective,
is already gone.

Amends 931f8d84ac

Change-Id: Idf31f9eb761606330184a9f767c7f63a4f1167dc
Reviewed-by: hjk <hjk@qt.io>

src/plugins/valgrind/callgrindtool.cpp

@@ -768,6 +768,9 @@ void CallgrindTool::clearTextMarks()
 
 void CallgrindTool::engineFinished()
 {
+    if (m_flatView == nullptr) // Happens on shutdown when memcheck is still running.
+        return;
+
     m_toolBusy = false;
     updateRunActions();