patch 9.0.1357: using null_object results in an internal error

Problem:    Using null_object results in an internal error. (Ernie Rael)
Solution:   Add instructions for pushing an object and class. (closes #12044)
diff --git a/src/errors.h b/src/errors.h
index a5df8ed..f1c36ad 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3449,3 +3449,7 @@
 #endif
 EXTERN char e_cannot_use_color_none_did_you_mean_none[]
 	INIT(= N_("E1361: Cannot use color \"none\", did you mean \"NONE\"?"));
+#ifdef FEAT_EVAL
+EXTERN char e_cannot_use_non_null_object[]
+	INIT(= N_("E1362: Cannot use a non-null object"));
+#endif