patch 9.0.1317: crash when using an unset object variable

Problem:    Crash when using an unset object variable.
Solution:   Give an error instead. (closes #12005)
diff --git a/src/errors.h b/src/errors.h
index 336c187..0bfafac 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -1182,6 +1182,8 @@
 	INIT(= N_("E476: Invalid command"));
 EXTERN char e_invalid_command_str[]
 	INIT(= N_("E476: Invalid command: %s"));
+EXTERN char e_invalid_command_str_expected_str[]
+	INIT(= N_("E476: Invalid command: %s, expected %s"));
 EXTERN char e_no_bang_allowed[]
 	INIT(= N_("E477: No ! allowed"));
 EXTERN char e_dont_panic[]
@@ -3442,6 +3444,6 @@
 	INIT(= N_("E1358: Using \"super\" not in a child class"));
 EXTERN char e_cannot_define_new_function_in_abstract_class[]
 	INIT(= N_("E1359: Cannot define a \"new\" function in an abstract class"));
-EXTERN char e_invalid_command_str_expected_str[]
-	INIT(= N_("E476: Invalid command: %s, expected %s"));
+EXTERN char e_using_null_object[]
+	INIT(= N_("E1360: Using a null object"));
 #endif