TypeScript to show a validation function for PropertyPaneTextbox in SPFX which makes an async call. See http://cob-sp.com/SPFx-WP-Props1

asyncTextBoxValidationMethod.ts

private asyncTextBoxValidationMethod(value: string): Promise<string> {
    if (value !== undefined && value.length > 3) {
      var url = this.context.pageContext.web.absoluteUrl + `/_api/web/lists?$filter=Hidden eq false`;

      return this.fetchLists(url).then((response) => {
          var lists: ISPList[] = response.value;
          var foundList: boolean = false;
          lists.forEach((list: ISPList) => {
              if (value === list.Title) {
                  foundList = true;
              }
          });

          if (!foundList) {
            // resolve promise with error message to display..
            return Promise.resolve("Value entered did not match a list in this site!");
          }

          // otherwise resolve promise with an empty string..
          return Promise.resolve("");
        });
      }
  }