message: Add guards to check that `TaskQueue`s have at least one processor
diff: @@ -218,7 +218,7 @@ export default class TaskQueue implements Evented {
  this._cancel();
  this._tasks.shift();
  let processor = this._processors.shift();
  - if (!processor.settled) {
  + if (processor !== undefined && !processor.settled) {
  processor.reject(
  e || new Error('Processing cancelled via `TaskQueue#skip`')
  );
  @@ -262,7 +262,7 @@ export default class TaskQueue implements Evented {
  this._cancel();
  task = this._tasks.shift();
  let processor = this._processors.shift();
  - if (!processor.settled) {
  + if (processor !== undefined && !processor.settled) {
  processor.reject(
  e || new Error('Processing cancelled via `TaskQueue#shift`')
  );
  