patch 9.0.1402: crash when using null_class

Problem:    Crash when using null_class.
Solution:   Give an error when trying to use a null class.

Author: brammool

src/testdir/test_vim9_class.vim

@@ -203,6 +203,13 @@ def Test_returning_null_object()
   v9.CheckScriptSuccess(lines)
 enddef
 
+def Test_using_null_class()
+  var lines =<< trim END
+      @_ = null_class.member
+  END
+  v9.CheckDefExecAndScriptFailure(lines, ['E715:', 'E1363:'])
+enddef
+
 def Test_class_interface_wrong_end()
   var lines =<< trim END
       vim9script

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1402,
 /**/
     1401,
 /**/

src/vim9class.c

@@ -1262,6 +1262,12 @@ class_object_index(
 	cl = rettv->vval.v_object->obj_class;
     }
 
+    if (cl == NULL)
+    {
+	emsg(_(e_incomplete_type));
+	return FAIL;
+    }
+
     if (*name_end == '(')
     {
 	int on_class = rettv->v_type == VAR_CLASS;