Well this works for me. The top inject pops it up and the second clears it. If it doesn't work for you then perhaps you need to update the dashboard node.
[{"id":"6309027f.3ffcdc","type":"inject","z":"37f5b818.697af8","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"hello","payload":"xxx","payloadType":"str","x":230,"y":140,"wires":[["d0f2c477.fcd2e"]]},{"id":"53360a67.f8721c","type":"inject","z":"37f5b818.697af8","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"str","x":220,"y":240,"wires":[["d0f2c477.fcd2e"]]},{"id":"d0f2c477.fcd2e","type":"ui_toast","z":"37f5b818.697af8","position":"top right","displayTime":"1000","highlight":"","sendall":true,"outputs":0,"ok":"OK","cancel":"","raw":false,"topic":"","name":"","x":400,"y":200,"wires":[]}]
