Valgrind: Fix a crash on shutdown

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

Change-Id: I5f872ee2f14c2d864ac45a248c2d267d28677661
Reviewed-by: hjk <hjk@qt.io>

src/plugins/valgrind/memchecktool.cpp

@@ -1108,6 +1108,9 @@ int MemcheckTool::updateUiAfterFinishedHelper()
 
 void MemcheckTool::engineFinished()
 {
+    if (m_errorView == nullptr) // Happens on shutdown when memcheck is still running.
+        return;
+
     m_toolBusy = false;
     updateRunActions();