message: Prevent crash in `isDisposable()`
diff: @@ -10,6 +10,9 @@ import {Disposable} from '@extraterm/extraterm-extension-api';
  * Type guard to detecting objects which support the Disposable interface.
  */
  export function isDisposable(it: any): it is Disposable {
  + if (it == null) {
  + return false;
  + }
  return 'dispose' in it;
  }
  