No credentials found for credentialsId

## Issue

### Context
- **Gitlab plugin version**: 1.5.13
- **Gitlab version**: 12.3.5 (2417d5becc7)
- **Jenkins version**: 2.190.1
- **Job type**: Freestyle

### Logs & Traces
```
17:16:32 ERROR: Build step failed with exception
17:16:32 java.lang.IllegalStateException: No credentials found for credentialsId:
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnection.getApiToken(GitLabConnection.java:134)
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnection.getClient(GitLabConnection.java:116)
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnectionConfig.getClient(GitLabConnectionConfig.java:92)
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnectionProperty.getClient(GitLabConnectionProperty.java:37)
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnectionProperty.getClient(GitLabConnectionProperty.java:45)
17:16:32 at com.dabsquared.gitlabjenkins.publisher.MergeRequestNotifier.perform(MergeRequestNotifier.java:31)
17:16:32 at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
17:16:32 at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:741)
17:16:32 at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:690)
17:16:32 at hudson.model.Build$BuildExecution.post2(Build.java:186)
17:16:32 at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:635)
17:16:32 at hudson.model.Run.execute(Run.java:1840)
17:16:32 at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
17:16:32 at hudson.model.ResourceController.execute(ResourceController.java:97)
17:16:32 at hudson.model.Executor.run(Executor.java:429)
17:16:32 Build step 'Add note with build status on GitLab merge requests' marked build as failure
17:16:32 ERROR: Build step failed with exception
17:16:32 java.lang.IllegalStateException: No credentials found for credentialsId:
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnection.getApiToken(GitLabConnection.java:134)
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnection.getClient(GitLabConnection.java:116)
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnectionConfig.getClient(GitLabConnectionConfig.java:92)
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnectionProperty.getClient(GitLabConnectionProperty.java:37)
17:16:32 at com.dabsquared.gitlabjenkins.connection.GitLabConnectionProperty.getClient(GitLabConnectionProperty.java:45)
17:16:32 at com.dabsquared.gitlabjenkins.publisher.MergeRequestNotifier.perform(MergeRequestNotifier.java:31)
17:16:32 at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
17:16:32 at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:741)
17:16:32 at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:690)
17:16:32 at hudson.model.Build$BuildExecution.post2(Build.java:186)
17:16:32 at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:635)
17:16:32 at hudson.model.Run.execute(Run.java:1840)
17:16:32 at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
17:16:32 at hudson.model.ResourceController.execute(ResourceController.java:97)
17:16:32 at hudson.model.Executor.run(Executor.java:429)
17:16:32 Build step 'Add vote for build status on GitLab merge requests' marked build as failure
```

### Problem description

I added "Add note with build status on GitLab merge requests" and "Add vote for build status on GitLab merge requests" to my Freestyle Job as Post-build Actions. But when the job gets triggered by a comment in the Merge request I receive the error above. The project itself gets cloned though via the connection and the build itself is executed.
Am I missing anything here or is that a bug?