NPE When creating webhooks for Organisation folders

Somewhere after version 1.4.1 a NullPointer Exception was introduced, when creating webHooks for subprojects within an organisation folder.
To reproduce it, scan for subprojects within an organisation folder and confuigure that webhooks should be created.

Here is the log for this behavior:
[Thu Jun 10 14:26:37 CEST 2021] Starting organization scan...
[Thu Jun 10 14:26:37 CEST 2021] Updating actions...
Group URL: ...
[Thu Jun 10 14:26:40 CEST 2021] Consulting GitLab Group
Checking projects...

Checking project ...
ERROR: [Thu Jun 10 14:26:45 CEST 2021] Could not fetch sources from navigator io.jenkins.plugins.gitlabbranchsource.GitLabSCMNavigator@d87e8c0a
[Thu Jun 10 14:26:45 CEST 2021] Finished organization scan. Scan took 7.7 sec
FATAL: Failed to recompute children of ...
java.lang.NullPointerException
at io.jenkins.plugins.gitlabbranchsource.GitLabSCMNavigator.visitSources(GitLabSCMNavigator.java:274)
at jenkins.branch.OrganizationFolder.computeChildren(OrganizationFolder.java:542)
at com.cloudbees.hudson.plugins.folder.computed.ComputedFolder.updateChildren(ComputedFolder.java:278)
at com.cloudbees.hudson.plugins.folder.computed.FolderComputation.run(FolderComputation.java:165)
at jenkins.branch.OrganizationFolder$OrganizationScan.run(OrganizationFolder.java:1031)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:429)
Finished: FAILURE