The reason for the exception is that when the IsBusy of the indicator changes, the Content is added to and removed from the visual tree. Because it is data-bound, the chart immediately tries to notify its native counterpart that an update is necessary, and the iOS control is looked up. However, sometimes, the old iOS control is found, which has been disposed.
A possible workaround is not to put content inside the busy indicator. Use the busy indicator as a simple overlay.

Available in the R3 2018 SP release.