prevent dangling pointers by clearing the active assembly after it was deleted

src/Mod/Assembly/Gui/AppAssemblyGuiPy.cpp

@@ -79,6 +79,16 @@ static PyObject * setActiveAssembly(PyObject *self, PyObject *args)
     Py_Return;
 }
 
+static PyObject* clearActiveAssembly(PyObject* self,  PyObject* args) {
+
+    ActiveAsmObject = 0;
+    ActiveGuiDoc    =0;
+    ActiveAppDoc    =0;
+    ActiveVp        =0;
+    
+    Py_Return;
+}
+
 /* module functions */
 static PyObject* getActiveAssembly(PyObject* self, PyObject* args)
 {
@@ -95,9 +105,14 @@ static PyObject * getActiveAssembly(PyObject *self, PyObject *args)
 /* registration table  */
 struct PyMethodDef AssemblyGui_Import_methods[] = {
     {   "setActiveAssembly"       ,setActiveAssembly      ,METH_VARARGS,
-     "setActiveAssembly(AssemblyObject) -- Set the Assembly object in work."},
+        "setActiveAssembly(AssemblyObject) -- Set the Assembly object in work."
+    },
     {   "getActiveAssembly"       ,getActiveAssembly      ,METH_VARARGS,
-     "getActiveAssembly() -- Returns the Assembly object in work."},
+        "getActiveAssembly() -- Returns the Assembly object in work."
+    },
+    {   "clearActiveAssembly"       ,clearActiveAssembly      ,METH_VARARGS,
+        "clearActiveAssembly() -- Removes the current active Assembly as object in work"
+    },
 
     {NULL, NULL}                   /* end of table marker */
 };

src/Mod/Assembly/Gui/ViewProviderAssembly.cpp

@@ -35,6 +35,8 @@
 
 using namespace AssemblyGui;
 
+extern Assembly::ItemAssembly* ActiveAsmObject;
+
 PROPERTY_SOURCE(AssemblyGui::ViewProviderItemAssembly,AssemblyGui::ViewProviderItem)
 
 ViewProviderItemAssembly::ViewProviderItemAssembly()
@@ -43,6 +45,8 @@ ViewProviderItemAssembly::ViewProviderItemAssembly()
 
 ViewProviderItemAssembly::~ViewProviderItemAssembly()
 {
+    if(getObject() == ActiveAsmObject)
+        Gui::Command::doCommand(Gui::Command::Doc,"AssemblyGui.clearActiveAssembly()");
 }
 
 bool ViewProviderItemAssembly::doubleClicked(void)