Using this gives us warnings in webpack based karma tests, possibly t…

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Using this gives us warnings in webpack based karma tests, possibly to the differently executed lifecycle in the test environment

• this slider is only created on mounted
• if younted is not created, no slider on this, thus destroy is not defined.

ERROR LOG: '[Vue warn]: Error in beforeDestroy hook: "TypeError: null is not an object (evaluating 'this.slider.destroy')"