Sketcher: Bug fix: Building Vertex Index on Restore

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

The bug was introduced by myself here:
47c0859

[code]
void SketchObject::onDocumentRestored()
 {
     try {
-        rebuildExternalGeometry();
+        if(Support.getValue()) {
+            validateExternalLinks();
+            rebuildExternalGeometry();
+        }
         Constraints.acceptGeometry(getCompleteGeometry());
     }
[/code]

The issue is that "rebuildExternalGeometry" calls "rebuildVertexIndex". So for any sketch not having external geometry, if you open it, select everything and hit del, you will have a crash because
I effectively removed the creation of the index in this dossier. You won't have one if you do anything rebuilding the index before hitting del, like creating geometry, though.

src/Mod/Sketcher/App/SketchObject.cpp

@@ -3727,6 +3727,9 @@ void SketchObject::onDocumentRestored()
             validateExternalLinks();
             rebuildExternalGeometry();            
         }
+        else {
+            rebuildVertexIndex();
+        }
         Constraints.acceptGeometry(getCompleteGeometry());
     }
     catch (...) {