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
 

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1370,
 /**/
     1369,
 /**/

src/vim9execute.c

@@ -5321,6 +5321,13 @@ exec_instructions(ectx_T *ectx)
 		    }
 
 		    object_T *obj = tv->vval.v_object;
+		    if (obj == NULL)
+		    {
+			SOURCING_LNUM = iptr->isn_lnum;
+			emsg(_(e_using_null_object));
+			goto on_error;
+		    }
+
 		    int idx;
 		    if (iptr->isn_type == ISN_GET_OBJ_MEMBER)
 			idx = iptr->isn_arg.number;