Sketcher: Fix detach observer

src/Mod/Sketcher/Gui/ViewProviderSketch.cpp

@@ -398,6 +398,7 @@ ViewProviderSketch::ViewProviderSketch()
 ViewProviderSketch::~ViewProviderSketch()
 {
     delete rubberband;
+    unsubscribeToParameters();
 }
 
 void ViewProviderSketch::slotUndoDocument(const Gui::Document& /*doc*/)
@@ -3798,6 +3799,12 @@ void ViewProviderSketch::subscribeToParameters()
      hGrp->Attach(this);
 }
 
+void ViewProviderSketch::unsubscribeToParameters()
+{
+     ParameterGrp::handle hGrp = App::GetApplication().GetParameterGroupByPath("User parameter:BaseApp/Preferences/View");
+     hGrp->Detach(this);
+}
+
 void ViewProviderSketch::updateInventorNodeSizes()
 {
     assert(edit);

src/Mod/Sketcher/Gui/ViewProviderSketch.h

@@ -311,6 +311,8 @@ protected:
     void initItemsSizes();
     /// subscribe to parameter groups as an observer
     void subscribeToParameters();
+    /// unsubscribe to parameter groups as an observer
+    void unsubscribeToParameters();
     /// updates the sizes of the edit mode inventor node
     void updateInventorNodeSizes();