I think that you are adding the template in the wrong location. At your widget php file, uncomment the following line inside the init function:
$this->template( array( \IPS\Theme::i()->getTemplate( 'widgets', $this->app, 'front' ), $this->key ) );
Then inside the /dev/html folder, create a new folder called: front and inside that folder create another folder called: widgets and inside that folder create your template file ending in extension phtml. Hope it helps.