Fix crash when filtering out warnings from issues pane

Do not send endRemoveRows if we never sent beginRemoveRows.
Fix up of 702d6a6914

Task-number: QTCREATORBUG-20741
Change-Id: If22a18e6426c8b4041924a170d5927ab9d11ccdf
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/taskmodel.h

@@ -165,6 +165,7 @@ private:
     void updateMapping() const;
     bool filterAcceptsTask(const Task &task) const;
 
+    bool m_beginRemoveRowsSent = false;
     bool m_includeUnknowns;
     bool m_includeWarnings;
     bool m_includeErrors;