Toast: Unexpected error: ReferenceError: _a is not defined

[ココロ]

  // ...
  static create(props: ToasterFeatureProps, container = ReactHelper.document?.body) { // <----- optional chaining in function default parameter
    const containerElement = ReactHelper.document?.createElement("div");
  // ...

reference: microsoft/TypeScript#36295

[ココロ]

fixed in ca56e92