Fix some problems with ArchFence

The commit that introduces the colorization option added some problems.
First the "sectionFaceNumbers" property is only set when an execute is
performed. When this does not happen, an exception is thrown when the
document is saved that contains a fence created with an older version
of FreeCAD.

Also the new Props on the ViewProvider where not set because the
"onDocumentRestored" is not reckognized for ViewProviders.

src/Mod/Arch/ArchFence.py

@@ -59,7 +59,10 @@ class _Fence(ArchComponent.Component):
         self.Type = "Fence"
 
     def __getstate__(self):
-        return (self.sectionFaceNumbers)
+        if hasattr(self, 'sectionFaceNumbers'):
+            return (self.sectionFaceNumbers)
+        
+        return None
 
     def __setstate__(self, state):
         if state is not None and isinstance(state, tuple):
@@ -237,9 +240,11 @@ class _ViewProviderFence(ArchComponent.ViewProviderComponent):
             vobj.addProperty("App::PropertyBool", "UseOriginalColors", "Fence", QT_TRANSLATE_NOOP(
                 "App::Property", "When true, the fence will be colored like the original post and section."))
 
-    def onDocumentRestored(self, vobj):
+    def attach(self, vobj):
         self.setProperties(vobj)
 
+        return super().attach(vobj)
+
     def getIcon(self):
         import Arch_rc