if (!this.props.readOnly) {
        const method = this._handler && this._handler[eventName];
        if (method) {
          if (flushControlled) {
            flushControlled(() => method(this, e));
          } else {
            method(this, e);
          }
        }
      }
Build a method that will pass the event to the specified handler method.
This allows us to look up the correct handler function for the current
editor mode, if any has been specified.