Creating import configuration throws NPE

[matthias-ronge]

Describe the bug
I tried to configure the Kalliope import. This error message occurred when saving. The import configuration was not saved.

java.lang.NullPointerException
    at org.kitodo.production.forms.validators.ImportConfigurationParentMappingValidator.validate(ImportConfigurationParentMappingValidator.java:51)
    at org.kitodo.production.forms.validators.ImportConfigurationParentMappingValidator.validate(ImportConfigurationParentMappingValidator.java:27)
    at javax.faces.component._ComponentUtils.callValidators(_ComponentUtils.java:291)
    at javax.faces.component.UIInput.validateValue(UIInput.java:489)
    at javax.faces.component.UISelectOne.validateValue(UISelectOne.java:155)
    at org.primefaces.component.selectonemenu.SelectOneMenu.validateValue(SelectOneMenu.java:198)
    at javax.faces.component.UIInput.validate(UIInput.java:746)
    at javax.faces.component.UIInput.processValidators(UIInput.java:293)
    at javax.faces.component.UISelectOne.processValidators(UISelectOne.java:117)
    at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1458)
    at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1458)
    at javax.faces.component.UIComponentBase.processValidators(UIComponentBase.java:1458)
    ...

To Reproduce
Steps to reproduce the behavior:

1. Fill in data, see screenshots below
2. Click on 'Save'
3. See error

Expected behavior
The process should be saved, or an understandable error message should appear if there is a problem.

Release
3.6.2

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Firefox 124.0.1

Additional context

[BartChris]

Maybe related: #5647