Sketcher/Part: Python handling of GeometryExtensions without a Python counterpart

=================================================================================

Some geometry extensions do not provide a PyObject as they do not have a Python counterpart
as it would serve no purpose to have it.

This commit handles this situation making sure to provide the right error to Python
or to ignore the extension where appropriate.

src/Mod/Part/App/GeometryPyImp.cpp

@@ -261,6 +261,10 @@ PyObject* GeometryPy::getExtensionOfType(PyObject *args)
                 PyErr_SetString(PartExceptionOCCError, "Geometry extension does not exist anymore.");
                 return 0;
             }
+            catch(Base::NotImplementedError) {
+                PyErr_SetString(Part::PartExceptionOCCError, "Geometry extension does not implement a Python counterpart.");
+                return 0;
+            }
         }
         else
         {
@@ -296,6 +300,10 @@ PyObject* GeometryPy::getExtensionOfName(PyObject *args)
             PyErr_SetString(PartExceptionOCCError, "Geometry extension does not exist anymore.");
             return 0;
         }
+        catch(Base::NotImplementedError) {
+            PyErr_SetString(Part::PartExceptionOCCError, "Geometry extension does not implement a Python counterpart.");
+            return 0;
+        }
 
     }
 
@@ -408,7 +416,7 @@ PyObject* GeometryPy::getExtensions(PyObject *args)
     try {
         const std::vector<std::weak_ptr<const GeometryExtension>> ext = this->getGeometryPtr()->getExtensions();
 
-        PyObject* list = PyList_New(ext.size());
+        PyObject* list = PyList_New(0);
 
         for (std::size_t i=0; i<ext.size(); ++i) {
 
@@ -417,10 +425,17 @@ PyObject* GeometryPy::getExtensions(PyObject *args)
 
             if(p) {
                 // we create a python copy and add it to the list
-                Py::Tuple tuple;
-                PyObject* cpy = static_cast<GeometryExtensionPy *>(p->getPyObject())->copy(tuple.ptr());
 
-                PyList_SetItem( list, i, cpy);
+                try {
+                    Py::Tuple tuple;
+                    PyObject* cpy = static_cast<GeometryExtensionPy *>(p->getPyObject())->copy(tuple.ptr());
+
+                    PyList_Append( list, cpy);
+                    Py_DECREF(cpy);
+                }
+                catch(Base::NotImplementedError) {
+                    // silently ignoring extensions not having a Python object
+                }
             }
         }