patch 9.0.1370: crash when using a NULL object

Problem:    Crash when using a NULL object. (Ernie Rael)
Solution:   Check for NULL and give an error message. (closes #12083)

src/testdir/test_vim9_class.vim

@@ -235,6 +235,24 @@ def Test_object_not_set()
   END
   v9.CheckScriptFailure(lines, 'E1360:')
 
+  lines =<< trim END
+      vim9script
+
+      class Class
+          this.id: string
+          def Method1()
+              echo 'Method1' .. this.id
+          enddef
+      endclass
+
+      var obj: Class
+      def Func()
+          obj.Method1()
+      enddef
+      Func()
+  END
+  v9.CheckScriptFailure(lines, 'E1360:')
+
   lines =<< trim END
       vim9script