Hey João,
If I'm not mistaken the property <actionVariables> is only available for 10.10 or higher.
To fix your problem try this:
<property key="random" type="decimal">
<caption>Random number between 0 and 1</caption>
<description>A decimal value generated by the widget</description>
</property>
<!-- Action property -->
<property key="onClick" type="action">
<caption>On Click</caption>
<description>Action to be executed when the button is clicked</description>
</property>
And then, you can call it using the OnClick property on Typescript
If this helped, click to accept :)
Best Regards,
Ricardo Pereira