Fix NullReferenceException in SymbolCollector

src/Libraries/NRefactory/ICSharpCode.NRefactory/Analysis/SymbolCollector.cs

@@ -129,12 +129,15 @@ namespace ICSharpCode.NRefactory.Analysis
 				}
 
 				if (member.DeclaringType.Kind == TypeKind.Interface) {
-					foreach (var derivedType in g.GetNode (member.DeclaringTypeDefinition).DerivedTypes) {
+					var declaringTypeNode = g.GetNode(member.DeclaringTypeDefinition);
+					if (declaringTypeNode != null) {
+						foreach (var derivedType in declaringTypeNode.DerivedTypes) {
 							var mem = SearchMember (derivedType.TypeDefinition, member);
 							if (mem != null)
 								symbols.Add (mem);
 						}
 					}
+				}
 
 
 				if (IncludeOverloads) {