Avoid a NullReferenceException in case of error

Author: obones

project/core/Config/preprocessor/ElementProcessors/ImportProcessor.cs

@@ -28,6 +28,14 @@ public override IEnumerable< XNode > Process(XNode node)
             }
             Type type;
             type = assembly != null ? assembly.GetType( type_ref ) : Type.GetType( type_ref );
+            
+            if (type == null)
+            {
+                throw ImportException.CreateException(
+                    "Imported type '{0}' could not be found in '{1}'",
+                    type_ref, assembly_loc);
+            }
+            
             var processor = Activator.CreateInstance( type, _Env ) as IElementProcessor;
             if ( processor == null )
             {