UI changes on background threads are not allowed. 

2018-05-04 15:26:09.022504-0400 NYPLAudiobookToolkitDemoApp[750:187770] [reports] Main Thread Checker: UI API called on a background thread: -[UIActivityIndicatorView stopAnimating]PID: 750, TID: 187770, Thread name: (none), Queue name: org.nypl.labs.NYPLAudiobookToolkit.FindawayPlayer, QoS: 0Backtrace:4   NYPLAudiobookToolkit                0x0000000104f370ac T020NYPLAudiobookToolkit29AudiobookPlayerViewControllerC6playeryAA0D0_p_AA15ChapterLocationC18didBeginPlaybackOftF + 2005   NYPLAudiobookToolkit                0x0000000104f37128 _T020NYPLAudiobookToolkit29AudiobookPlayerViewControllerC6playeryAA0D0_p_AA15ChapterLocationC18didBeginPlaybackOftFTo + 966   NYPLAudiobookToolkit                0x0000000104f25aec _T020NYPLAudiobookToolkit14FindawayPlayerC28notifyDelegatesOfPlaybackFor33_9E5169E64ED8C8506EEB227D991BEF62LLyAA15ChapterLocationC7chapter_tFyAA0D8Delegate_pcfU + 967   NYPLAudiobookToolkit                0x0000000104f25b2c T020NYPLAudiobookToolkit14PlayerDelegate_ps5Error_pIgxzo_AaB_psAC_pIgizo_TR + 368   NYPLAudiobookToolkit                0x0000000104f2a3b8 _T020NYPLAudiobookToolkit14PlayerDelegate_ps5Error_pIgxzo_AaB_psAC_pIgizo_TRTA.109 + 409   libswiftCore.dylib                  0x000000010552fba8 _T0s8SequencePsE7forEachyy7ElementQzKcKFTf4gn_n + 35610  libswiftCore.dylib                  0x0000000105467424 _T0s8SequencePsE7forEachyy7ElementQzKcKFTm + 2011  libswiftCore.dylib                  0x000000010536cd0c _T0s8SequencePsE7forEachyy7ElementQzKcKF + 2012  NYPLAudiobookToolkit                0x0000000104f2596c _T020NYPLAudiobookToolkit14FindawayPlayerC28notifyDelegatesOfPlaybackFor33_9E5169E64ED8C8506EEB227D991BEF62LLyAA15ChapterLocationC7chapter_tF + 39613  NYPLAudiobookToolkit                0x0000000104f24cac _T020NYPLAudiobookToolkit14FindawayPlayerC22loadAndRequestPlayback33_9E5169E64ED8C8506EEB227D991BEF62LLyAA15ChapterLocationCF + 136814  NYPLAudiobookToolkit                0x0000000104f2546c _T020NYPLAudiobookToolkit14FindawayPlayerC20performDelayedAction33_9E5169E64ED8C8506EEB227D991BEF62LLyyFyycfU2 + 20415  NYPLAudiobookToolkit                0x0000000104f242f4 T020NYPLAudiobookToolkit14FindawayPlayerC20performDelayedAction33_9E5169E64ED8C8506EEB227D991BEF62LLyyF25enqueueEngineManipulationL_yyF022attemptToPerformQueuedpQ0L_yyF + 30416  NYPLAudiobookToolkit                0x0000000104f24504 _T020NYPLAudiobookToolkit14FindawayPlayerC20performDelayedAction33_9E5169E64ED8C8506EEB227D991BEF62LLyyF25enqueueEngineManipulationL_yyFyycfU + 2017  NYPLAudiobookToolkit                0x0000000104f21fb8 _T0Ieg_IeyB_TR + 5218  libdispatch.dylib                   0x0000000105d79220 _dispatch_client_callout + 1619  libdispatch.dylib                   0x0000000105d864d8 _dispatch_continuation_pop + 58820  libdispatch.dylib                   0x0000000105d7aee8 _dispatch_source_invoke + 136821  libdispatch.dylib                   0x0000000105d87ca4 _dispatch_queue_serial_drain + 29222  libdispatch.dylib                   0x0000000105d7c730 _dispatch_queue_invoke + 32823  libdispatch.dylib                   0x0000000105d88dd8 _dispatch_root_queue_drain_deferred_wlh + 35224  libdispatch.dylib                   0x0000000105d8febc _dispatch_workloop_worker_thread + 67625  libsystem_pthread.dylib             0x0000000183587e70 _pthread_wqthread + 86026  libsystem_pthread.dylib             0x0000000183587b08 start_wqthread + 4