amp-bind: Default value for [text] does not match first expression result (null). This can result in unexpected behavior after the next state change.​​​

To fix this you need to define default value of the [text] like this :

<div [text]="(SetState== null ? 'Default Text goes here' : SetState)" >Default Text goes  here</div>

Then you can replace the text like this :

<button on="tap:AMP.setState({SetState: 'New Defautl text...'})">Set text</button>