Unable to delete the Default Configuration Scheme for a custom field without a project associated

Summary

A NullArgumentException is seen when trying to delete the "Default Configuration Scheme" from a custom field having a non-global scheme context without a project associated.
If you create a non-global Custom Field Configuration scheme and then delete the project associated with it, will cause the error when trying to remove the default global scheme.

Steps to Reproduce

1. Create a Jira project;
2. Create a custom field;
3. Go to the custom field configuration page;
4. Add a new context associated with the project you've just created;
5. Delete the project you've just created;
6. Try to delete the "Default Configuration Scheme" for the custom field;

Expected Results

It was expected to have the "Default Configuration Scheme" deleted, or maybe show a friendly error message explaining why it can't be deleted if that is the case.

Actual Results

You'll see the following exception.

com.atlassian.jira.util.dbc.Assertions$NullArgumentException: clause should not be null!
    at com.atlassian.jira.util.dbc.Assertions.notNull(Assertions.java:25) [jira-api-7.12.1.jar:?]
    at com.atlassian.jira.jql.builder.DefaultJqlClauseBuilder.addClause(DefaultJqlClauseBuilder.java:634) [classes/:?]
    at com.atlassian.jira.web.action.admin.customfields.CustomFieldContextConfigHelperImpl.doesRemovingSchemeFromCustomFieldAffectIssues(CustomFieldContextConfigHelperImpl.java:101) [classes/:?]
    at com.atlassian.jira.web.action.admin.customfields.ManageConfigurationScheme.doRemove(ManageConfigurationScheme.java:220) [classes/:?]
...

Workaround

1. Create a dummy project;
2. Go to the custom field configuration page;
3. Add the dummy project to the configuration scheme without project;
4. Delete the default configuration;
5. You can delete de dummy project if you want.