add protected default constructor of SelectionFilterGate to simplifiy writing of sub-classes

src/Gui/SelectionFilter.cpp

@@ -62,6 +62,11 @@ SelectionFilterGate::SelectionFilterGate(SelectionFilter* filter)
     Filter = filter;
 }
 
+SelectionFilterGate::SelectionFilterGate()
+{
+    Filter = nullptr;
+}
+
 SelectionFilterGate::~SelectionFilterGate()
 {
     delete Filter;

src/Gui/SelectionFilter.h

@@ -108,6 +108,9 @@ public:
     ~SelectionFilterGate();
     virtual bool allow(App::Document*,App::DocumentObject*, const char*);
 
+protected:
+    SelectionFilterGate();
+
 protected:
     SelectionFilter *Filter;
 };

src/Mod/Part/Gui/TaskFaceColors.cpp

@@ -75,7 +75,7 @@ namespace PartGui {
         const App::DocumentObject* object;
     public:
         FaceSelection(const App::DocumentObject* obj)
-            : Gui::SelectionFilterGate((Gui::SelectionFilter*)0), object(obj)
+            : Gui::SelectionFilterGate(), object(obj)
         {
         }
         bool allow(App::Document* /*pDoc*/, App::DocumentObject*pObj, const char*sSubName)