AttachEngine: fix crash when referenced objects get deleted

... by verifying if the pointers equal to objects contained in all open
documents. Not terribly good, but I can't think of a situation where
doing this search might cause trouble.

src/Mod/Part/App/AttachEnginePyImp.cpp

@@ -111,6 +111,7 @@ Py::Object AttachEnginePy::getReferences(void) const
 {
     try {
         AttachEngine &attacher = *(this->getAttachEnginePtr());
+        AttachEngine::verifyReferencesAreSafe(attacher.references);
         return Py::Object(attacher.references.getPyObject(),true);
     } ATTACHERPY_STDCATCH_ATTR;
 }
@@ -523,6 +524,7 @@ PyObject* AttachEnginePy::writeParametersToFeature(PyObject* args)
         }
         Part::AttachableObject* feat = static_cast<Part::AttachableObject*>(dobj);
         const AttachEngine &attacher = *(this->getAttachEnginePtr());
+        AttachEngine::verifyReferencesAreSafe(attacher.references);
         feat->Support.Paste(attacher.references);
         feat->MapMode.setValue(attacher.mapMode);
         feat->MapReversed.setValue(attacher.mapReverse);

src/Mod/Part/App/Attacher.cpp

@@ -61,6 +61,8 @@
 #include "Attacher.h"
 #include <Base/Console.h>
 #include <App/OriginFeature.h>
+#include <App/Application.h>
+#include <App/Document.h>
 
 using namespace Part;
 using namespace Attacher;
@@ -749,6 +751,7 @@ void AttachEngine::readLinks(const App::PropertyLinkSubList &references,
                              std::vector<TopoDS_Shape> &storage,
                              std::vector<eRefType> &types)
 {
+    verifyReferencesAreSafe(references);
     const std::vector<App::DocumentObject*> &objs = references.getValues();
     const std::vector<std::string> &sub = references.getSubValues();
     geofs.resize(objs.size());
@@ -830,6 +833,23 @@ void AttachEngine::throwWrongMode(eMapMode mmode)
     throw Base::Exception(errmsg.str().c_str());
 }
 
+void AttachEngine::verifyReferencesAreSafe(const App::PropertyLinkSubList &references)
+{
+    const std::vector<App::DocumentObject*> links =  references.getValues();
+    const std::vector<App::Document*> docs = App::GetApplication().getDocuments();
+    for(App::DocumentObject* lnk : links){
+        bool found = false;
+        for(App::Document* doc : docs){
+            if(doc->isIn(lnk)){
+                found = true;
+            }
+        }
+        if (!found){
+            throw Base::Exception("AttachEngine: verifyReferencesAreSafe: references point to deleted object.");
+        }
+    }
+}
+
 
 //=================================================================================
 

src/Mod/Part/App/Attacher.h

@@ -342,6 +342,12 @@ class PartExport AttachEngine : public Base::BaseClass
 
     static GProp_GProps getInertialPropsOfShape(const std::vector<const TopoDS_Shape*> &shapes);
 
+    /**
+     * @brief verifyReferencesAreSafe: checks if pointers in references still
+     * point to objects contained in open documents. This guarantees the links
+     * are valid. Throws Base::Exception if invalid links are found.
+     */
+    static void verifyReferencesAreSafe(const App::PropertyLinkSubList& references);
 
 public: //enums
     static const char* eMapModeStrings[];