Adding a field with no type using the content definition manager throws null exception.

[MikeAlhayek]

Describe the bug

When adding a field without specifiying the type of the field to the ContentDefinitionManager a null exception is thrown.

To Reproduce

Run the following migration

public class TestMigration : DataMigration
{
    private readonly IContentDefinitionManager _contentDefinitionManager;

    public TestMigration(IContentDefinitionManager contentDefinitionManager)
        => _contentDefinitionManager = contentDefinitionManager;
    

    public int Create()
    {
        _contentDefinitionManager.AlterPartDefinition("UserStoryInfoPart", part => part
            .WithDisplayName("User Story Info Part")
            .WithField("Description", part => part
                //.OfType("TextField") // without defining the type, we get a null exception.
                .WithEditor("TextArea")
            )
        );

        return 1;
    }
}

Expected behavior

Maybe an exception that is more description instead of a null exception.