title: Several flatmaps cause error on saving (Typescript)
body: <p>I have Angular component to upload files to server.</p>
  
  <p>Here is save method of it</p>
  
  <pre><code> private saveFileAndData(): void {
      this._propertyService
          .createOrEditProperty(this.property)
          .pipe(
              flatMap(r =&gt; this._fileUploadService.uploadFiles(this.getUploadUrl(r), this.files)),
              flatMap(r =&gt; this._fileUploadService.uploadFiles(this.getFloorPlanUploadUrl(r), this.floorPlans)),
              finalize(() =&gt; {
                  this.saving = false;
              })
          )
          .subscribe(() =&gt; {
              this.notify.info(this.l('SavedSuccessfully'));
              this.close();
              this.modalSave.emit(null);
          });
  }
  </code></pre>
  
  <p>When I was having one flat map, all was okay, now I need to load different files via save. So I have 2 flatMaps. But now I have this error </p>
  
  <p><a href="https://i.stack.imgur.com/EQpxM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/EQpxM.png" alt="enter image description here"></a></p>
  
  <p>If I leave just one flat map, all okay.</p>
  
  <p>Here is fileUpload service</p>
  
  <pre><code> @Injectable()
  export class FileUploadService {
      constructor(private _http: HttpClient) {}
  
      uploadFiles(url: string, filesToUpload: any[]): Observable&lt;any&gt; {
          if (filesToUpload.length) {
              const formData: FormData = new FormData();
              for (let file of filesToUpload) {
                  formData.append('file', file);
              }
              let headers = new HttpHeaders();
              return this._http.post(url, formData, { headers: headers });
          }
          return of(null);
      }
  }
  </code></pre>
  
  <p>How I can solve it?</p>