When hovering to the bottom left of a milestone and clicking & dragging to resize an error is thrown.
Uncaught TypeError: Cannot read property 'firstElementChild' of undefined
This breaks future sync attempts.
https://www.bryntum.com/examples/gantt/basic/
Go to the make backup milestone. Try to resize from the bottom left.
Thanks, reproduced. Ticket: https://github.com/bryntum/support/issues/2635