Account creation test flakes: Length of '[]' should be 1 but is 0

[php-coder]

After account creation an e-mail with activation link should be send  | FAIL |
[INFO] RequestHandlerClass from context returned com.github.tomakehurst.wiremock.http.AdminRequestHandler. Normalized mapped under returned 'null'
[INFO] RequestHandlerClass from context returned com.github.tomakehurst.wiremock.http.StubRequestHandler. Normalized mapped under returned 'null'
Length of '[]' should be 1 but is 0.

Example: https://travis-ci.org/php-coder/mystamps/jobs/591642237

[php-coder]

More details that were extracted from previous builds (https://travis-ci.org/php-coder/mystamps/jobs/585974928 and https://travis-ci.org/php-coder/mystamps/jobs/585974927):

After account creation an e-mail with activation link should be send 
[INFO] RequestHandlerClass from context returned com.github.tomakehurst.wiremock.http.AdminRequestHandler. Normalized mapped under returned 'null'
[INFO] RequestHandlerClass from context returned com.github.tomakehurst.wiremock.http.StubRequestHandler. Normalized mapped under returned 'null'
2019-09-17 09:34:52.580 [user:  ]  INFO 5597 --- [cTaskExecutor-2] r.m.w.s.m.ApiMailgunEmailSendingStrategy : Mailgun response code: 200
2019-09-17 09:34:52.580 [user:  ]  INFO 5597 --- [cTaskExecutor-2] r.m.w.s.m.ApiMailgunEmailSendingStrategy : Mailgun response body: {"id":"<20190507194211.0.B96DD98C0E6AAA9A@wiremock.localhost>","message":"Queued. Thank you."}
2019-09-17 09:34:52.580 [user:  ]  INFO 5597 --- [cTaskExecutor-2] r.m.web.feature.site.MailServiceImpl     : Email with activation code has been sent to coder@rock.home (lang: en)
| FAIL |
Length of '[]' should be 1 but is 0.

<kw library="BuiltIn" name="Log">
  <doc>Logs the given message with the given level.</doc>
  <arguments>
	<arg>${response.json}</arg>
  </arguments>
  <msg level="INFO" timestamp="20190917 09:34:43.735">{requests: [], requestJournalDisabled: false}</msg>
  <status endtime="20190917 09:34:43.745" starttime="20190917 09:34:43.726" status="PASS"></status>
</kw>

[php-coder]

I've just seen it there: https://travis-ci.org/php-coder/mystamps/jobs/602468042

[php-coder]

Perhaps, updating of Wiremock might help.

[php-coder]

It didn't help and the flake is still here (just seen in https://travis-ci.org/php-coder/mystamps/jobs/605758043):

After account creation an e-mail with activation link should be send  [INFO] JVM Runtime does not support Modules
[INFO] RequestHandlerClass from context returned com.github.tomakehurst.wiremock.http.StubRequestHandler. Normalized mapped under returned 'null'
[INFO] RequestHandlerClass from context returned com.github.tomakehurst.wiremock.http.AdminRequestHandler. Normalized mapped under returned 'null'
2019-10-31 22:33:33.279 [user:  ]  INFO 4889 --- [cTaskExecutor-2] r.m.w.s.m.ApiMailgunEmailSendingStrategy : Mailgun response code: 200
2019-10-31 22:33:33.282 [user:  ] DEBUG 4889 --- [cTaskExecutor-2] r.m.w.s.m.ApiMailgunEmailSendingStrategy : Mailgun response headers: {Content-Type=[application/json], Matched-Stub-Id=[c3defed0-cd0e-49f3-ba2a-99efce0f2019], Transfer-Encoding=[chunked], Server=[Jetty(9.4.20.v20190813)]}
2019-10-31 22:33:33.283 [user:  ]  INFO 4889 --- [cTaskExecutor-2] r.m.w.s.m.ApiMailgunEmailSendingStrategy : Mailgun response body: {"id":"<20190507194211.0.B96DD98C0E6AAA9A@wiremock.localhost>","message":"Queued. Thank you."}
2019-10-31 22:33:33.283 [user:  ]  INFO 4889 --- [cTaskExecutor-2] r.m.web.feature.site.MailServiceImpl     : Email with activation code has been sent to coder@rock.home (lang: en)
| FAIL |
Length of '[]' should be 1 but is 0.

[php-coder]

I've just seen it there: https://travis-ci.org/github/php-coder/mystamps/jobs/698000460