Timeline: destroy() fails if groupTemplate option is overwritten

Hi,

I am working with timeline (4.20) in chrome 59 on windows 7.

https://jsbin.com/puhekojozu/edit?html,js,output

Found another bug.

It seems that if you overwrite your own groupTemplate function and do not handle null groups, the destroy method fails. I do not think that destroy() should be calling the overwritten groupTemplate and if it should, perhaps it should reset all options to default first, so that this error does not occur. The corresponding jsbin is attached above.

Thanks