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/testdir/test_vim9_class.vim b/src/testdir/test_vim9_class.vim
index be9b18e..f1aadde 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -182,6 +182,21 @@
   v9.CheckScriptFailure(lines, 'E476: Invalid command: endclass, expected endinterface')
 enddef
 
+def Test_object_not_set()
+  var lines =<< trim END
+      vim9script
+
+      class State
+        this.value = 'xyz'
+      endclass
+
+      var state: State 
+      var db = {'xyz': 789}
+      echo db[state.value]
+  END
+  v9.CheckScriptFailure(lines, 'E1360:')
+enddef
+
 def Test_class_member_initializer()
   var lines =<< trim END
       vim9script