Added null checks to fix 'Uncaught TypeError: Cannot read property .. of null' errors when destroy() is called
Added null checks to fix 'Uncaught TypeError: Cannot read property .. of null' errors when destroy() is called

ffxsam/wavesurfer.js,katspaugh/wavesurfer.js,ale-grosselle/wavesurfer.js,katspaugh/wavesurfer.js,agamemnus/wavesurfer.js,ffxsam/wavesurfer.js,agamemnus/wavesurfer.js,ale-grosselle/wavesurfer.js