_createDeferredForProp (propertyName) {
    let deferred = this._promiseMap.get(propertyName)
    if (deferred == null) {
      deferred = pDefer()
      this._promiseMap.set(propertyName, deferred)
    }

    // @ts-expect-error - Need to fix generics
    return deferred
  }
Returns the existing promise for a property if it exists.
If not, one is created and set in the `_promiseMap`, then returned

@private
@template T
@param {ContextProperties} propertyName
@returns {pDefer.DeferredPromise<T>}