Fix NullReferenceException when mscorlib reference is missing.

src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/Implementations/ArrayReturnType.cs

@@ -122,13 +122,16 @@ namespace ICSharpCode.SharpDevelop.Dom
 		public override List<IProperty> GetProperties()
 		{
 			List<IProperty> l = base.GetProperties();
-			ArrayIndexer property = new ArrayIndexer(elementType, this.BaseType.GetUnderlyingClass());
+			IClass systemArray = this.BaseType.GetUnderlyingClass();
+			if (systemArray != null) {
+				ArrayIndexer property = new ArrayIndexer(elementType, systemArray);
 				IReturnType int32 = pc.SystemTypes.Int32;
 				for (int i = 0; i < dimensions; ++i) {
 					property.Parameters.Add(new DefaultParameter("index", int32, DomRegion.Empty));
 				}
 				property.Freeze();
 				l.Add(property);
+			}
 			return l;
 		}