[TD]fix segfault on confused selection

- failure in DrawDimHelper when both 2d & 3d geom selected

src/Mod/TechDraw/App/DrawDimHelper.cpp

@@ -336,7 +336,11 @@ DrawDimHelper::minMax3d(DrawViewPart* dvp, ReferenceVector references, int direc
     TopoDS_Compound comp;
     builder.MakeCompound(comp);
     for (auto& ref : references) {
-        builder.Add(comp, ref.getGeometry());
+        auto tempGeom = ref.getGeometry();
+        if (tempGeom.IsNull()) {
+            continue;
+        }
+        builder.Add(comp, tempGeom);
     }
     Base::Vector3d centroid = dvp->getOriginalCentroid();
     TopoDS_Shape centeredShape =//this result is a throw away. We will work with comp.

src/Mod/TechDraw/Gui/CommandCreateDims.cpp

@@ -2513,6 +2513,19 @@ void execExtent(Gui::Command* cmd, const std::string& dimType)
     TechDraw::DrawViewPart* partFeat =
         TechDraw::getReferencesFromSelection(references2d, references3d);
 
+    // if sticky selection is in use we may get confusing selections that appear to
+    // include both 2d and 3d geometry for the extent dim.
+    if (!references3d.empty())  {
+        for (auto& ref : references2d) {
+            if (!ref.getSubName().empty()) {
+                QMessageBox::warning(Gui::getMainWindow(),
+                    QObject::tr("Incorrect selection"),
+                    QObject::tr("Selection contains both 2d and 3d geometry"));
+                return;
+            }
+        }
+    }
+
     //Define the geometric configuration required for a extent dimension
     StringVector acceptableGeometry({"Edge"});
     std::vector<int> minimumCounts({1});

src/Mod/TechDraw/Gui/DimensionValidators.cpp

@@ -59,6 +59,7 @@ TechDraw::DrawViewPart* TechDraw::getReferencesFromSelection(ReferenceVector& re
                 //subName to a null string to avoid later misunderstandings.
                 ReferenceEntry ref(dvp, std::string());
                 references2d.push_back(ref);
+                continue;
             }
             for (auto& sub : selItem.getSubNames()) {
                 ReferenceEntry ref(dvp, sub);
@@ -192,7 +193,6 @@ DimensionGeometryType TechDraw::validateDimSelection3d(
         }
     }
 
-
     //check for invalid geometry descriptors in the subNames
     std::unordered_set<std::string> acceptableGeometrySet(acceptableGeometry.begin(),
                                                           acceptableGeometry.end());