fix(emqx_rule_engine): don't crash if we can't encode json

apps/emqx_rule_engine/src/emqx_rule_engine_api.erl

@@ -423,13 +423,21 @@ param_path_id() ->
 %% Internal functions
 %%------------------------------------------------------------------------------
 
-err_msg({RuleError, {_E, R, _S}}) when is_tuple(R) ->
-    emqx_misc:readable_error_msg(emqx_json:encode([{RuleError, element(1, R)}]));
-err_msg({RuleError, {_E, R, _S}}) ->
-    emqx_misc:readable_error_msg(emqx_json:encode([{RuleError, R}]));
+err_msg({RuleError, {_E, Reason, _S}}) ->
+    emqx_misc:readable_error_msg(encode_nested_error(RuleError, Reason));
 err_msg(Msg) ->
     emqx_misc:readable_error_msg(Msg).
 
+encode_nested_error(RuleError, Reason) when is_tuple(Reason) ->
+    encode_nested_error(RuleError, element(1, Reason));
+encode_nested_error(RuleError, Reason) ->
+    case emqx_json:safe_encode([{RuleError, Reason}]) of
+        {ok, Json} ->
+            Json;
+        _ ->
+            {RuleError, Reason}
+    end.
+
 format_rule_resp(Rules) when is_list(Rules) ->
     [format_rule_resp(R) || R <- Rules];
 format_rule_resp({Id, Rule}) ->