ZoneAware Error in Ionic Unit Tests - what is the root cause?

This suddenly appeared:

ERROR: Error{line: 3096, sourceURL: 'http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f', stack: 'ZoneAwareError@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:3096:28
resolvePromise@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2809:40
http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2860:31
invokeTask@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2495:36
onInvokeTask@http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:147059:49
invokeTask@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2494:48
runTask@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2262:57
drainMicroTaskQueue@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2690:42', originalStack: 'ZoneAwareError@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:3096:28
resolvePromise@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2809:40
http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2860:31
invokeTask@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2495:36
onInvokeTask@http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:147059:49
invokeTask@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2494:48
runTask@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2262:57
drainMicroTaskQueue@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2690:42', zoneAwareStack: 'http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2860:31 [ProxyZone]
onInvokeTask@http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:147059:49 [ProxyZone]
drainMicroTaskQueue@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2690:42 [<root>]', rejection: TypeError{stack: 'resolveComponent@http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:44699:54
_viewInit@http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:55749:78
_nextTrns@http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:55564:27
_queueTrns@http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:55521:23
http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:55318:36
invoke@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2462:31
onInvoke@http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:147035:45
invoke@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2461:40
run@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2222:49
http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2857:60
invokeTask@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2495:36
onInvokeTask@http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484:147059:49
invokeTask@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2494:48
runTask@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2262:57
drainMicroTaskQueue@http://localhost:9876/base/src/polyfills.ts?4d61da97b44e865713c30b37cbfe8766152efd0f:2690:42', line: 44699, sourceURL: 'http://localhost:9876/base/src/test.ts?dd3238f18d31f99d51ec41a64857770925c98484', __zone_symbol__currentTask: ZoneTask{_zone: ..., runCount: ..., _zoneDelegates: ..., _state: ..., type: ..., source: ..., data: ..., scheduleFn: ..., cancelFn: ..., callback: ..., invoke: ...}}, promise: ZoneAwarePromise{__zone_symbol__state: 0, __zone_symbol__value: TypeError{stack: ..., line: ..., sourceURL: ..., __zone_symbol__currentTask: ...}}, zone: Zone{_properties: Object{ProxyZoneSpec: ...}, _parent: Zone{_properties: ..., _parent: ..., _name: ..., _zoneDelegate: ...}, _name: 'ProxyZone', _zoneDelegate: ZoneDelegate{_taskCounts: ..., zone: ..., _parentDelegate: ..., _forkZS: ..., _forkDlgt: ..., _forkCurrZone: ..., _interceptZS: ..., _interceptDlgt: ..., _interceptCurrZone: ..., _invokeZS: ..., _invokeDlgt: ..., _invokeCurrZone: ..., _handleErrorZS: ..., _handleErrorDlgt: ..., _handleErrorCurrZone: ..., _scheduleTaskZS: ..., _scheduleTaskDlgt: ..., _scheduleTaskCurrZone: ..., _invokeTaskZS: ..., _invokeTaskDlgt: ..., _invokeTaskCurrZone: ..., _cancelTaskZS: ..., _cancelTaskDlgt: ..., _cancelTaskCurrZone: ..., _hasTaskZS: ..., _hasTaskDlgt: ..., _hasTaskDlgtOwner: ..., _hasTaskCurrZone: ...}}, task: ZoneTask{_zone: Zone{_properties: ..., _parent: ..., _name: ..., _zoneDelegate: ...}, runCount: 0, _zoneDelegates: null, _state: 'notScheduled', type: 'microTask', source: 'Promise.then', data: undefined, scheduleFn: undefined, cancelFn: null, callback: function () { ... }, invoke: function () { ... }}}