I am trying to create a global Widget. One of the fields in the Widget should be TreeDropdownField. I've managed to display it correctly by using the following code:

public function getCMSFields() { 
       $folder = new TreeDropdownField( 'FolderID', 'Show The Files From', 'Folder' );
       $folder->setFilterFunction( create_function( '$obj', 'return $obj->class == "Folder";' ) );
       return new FieldSet($folder);
}

when I am trying to view it and click on the drop down field, I am getting AJAX error that is thrown by sapphire\forms\Form.php, Form::callfieldmethod().

After executing the following URL
http://localhost/admin/?executeForm=EditForm&action_callfieldmethod=1&fieldName=Widget[44][FolderID]&ajax=1&SecurityID=32401&methodName=gettree&forceValues=0

ERROR:Error 256: Form::callfieldmethod() Field 'Widget[44][FolderID]' not found At l590 in .....\sapphire\forms\Form.php user_error(Form::callfieldmethod() Field 'Widget[44][FolderID]' not found,256) line 590 of Form.php Form->callfieldmethod(Array,Form) line 191 of Controller.php Controller->run(Array) line 76 of Director.php Director::direct(/admin/) line 104 of main.php

It seems that I can't have [ ]brackets as a fieldName parameter for this particular case.
Is there any way around this, it would be nice to have an option to display random images from the gallery

Did you find any fix to this issue yet? I'm just having the same problem...