fix exception raised on project load for invalid enums

src/App/PropertyStandard.cpp

@@ -377,6 +377,11 @@ const char ** PropertyEnumeration::getEnums(void) const
     return _enum.getEnums();
 }
 
+bool PropertyEnumeration::isValid(void) const
+{
+    return _enum.isValid();
+}
+
 void PropertyEnumeration::Save(Base::Writer &writer) const
 {
     writer.Stream() << writer.ind() << "<Integer value=\"" <<  _enum.getInt() <<"\"";

src/App/PropertyStandard.h

@@ -185,6 +185,9 @@ public:
 
     /// get the pointer to the enum list
     const char ** getEnums(void) const;
+
+    /// Returns true if the instance is in a usable state
+    bool isValid(void) const;
     //@}
 
     const char* getEditorName(void) const { return _editorTypeName.c_str(); }

src/Gui/ViewProviderDocumentObject.cpp

@@ -278,7 +278,7 @@ SoNode* ViewProviderDocumentObject::findFrontRootOfType(const SoType& type) cons
 
 void ViewProviderDocumentObject::setActiveMode()
 {
-    if (DisplayMode.getEnums()) {
+    if (DisplayMode.isValid()) {
         const char* mode = DisplayMode.getValueAsString();
         if (mode)
             setDisplayMode(mode);