Uncaught TypeError: Cannot read property 'eq' of undefined

Hi there!

The bug is already there in the actual jQ-UI v1.12.0-rc1 release.

e.widget._getSelectedItem	@	jquery-ui.min.js:11
(anonymous function)	@	jquery-ui.min.js:6
e.widget.refresh	@	jquery-ui.min.js:11
(anonymous function)	@	jquery-ui.min.js:6
(anonymous function)	@	jquery-ui.min.js:6
n.extend.each	@	jquery.min.js:2
n.fn.n.each	@	jquery.min.js:2
e.fn.(anonymous function)	@	jquery-ui.min.js:6
(anonymous function)	@	custom.js:2041
i	@	jquery.min.js:2
j.fireWith	@	jquery.min.js:2
z	@	jquery.min.js:4
(anonymous function)	@	jquery.min.js:4

Here my code for some debugging:

open: function(){
		$.getJSON('ajax/dokumente_dialogs_formitems.php?item=newfile_categories', function(data){
		    var $select = $("#DocumentsFile_Cat");
		    $select.appendTo("#DocumentsFile_Cat");
		    $.each([data], function(i, optgroups) {
			$.each(optgroups, function(groupName, options) {
			    var $optgroup = $("<optgroup>", {label: groupName});
			    $optgroup.appendTo($select);

			    $.each(options, function(j, option) {
				var $option = $("<option>", {text: option.text, value: option.value});
				$option.appendTo($optgroup);
			    });
			});
		    });
		});
		
		// Lade Daten in die Inputs
		$.getJSON('ajax/dokumente_dialogs_formitems.php?item=edit_document&fid='+FileID, function(data){
		    $('#DocumentsFileEdit #DocumentsFileForm #DocumentsFile_Cat').val(data[0].kid);
		   
		    
		    $('#DocumentsFileEdit #DocumentsFileForm #DocumentsFile_Name').val(data[0].name);
		    $('#DocumentsFileEdit #DocumentsFileForm #DocumentsFile_URL').val(data[0].url);
		    $('#DocumentsFileEdit #DocumentsFileForm #DocumentsFile_Pfad').val(data[0].pfad);
		    
		    if(data[0].versteckt == 1){
			$('#DocumentsFileEdit #DocumentsFileForm #DocumentsFile_Versteckt').prop("checked", true);
		    }
		    else{
			$('#DocumentsFileEdit #DocumentsFileForm #DocumentsFile_Versteckt').prop("checked", false);
		    } 
		    
		    $("#DocumentsFileEdit #DocumentsFileForm #DocumentsFile_Cat").selectmenu("refresh"); // 12.04.2016: Macht in jQuery UI 1.11.4 noch Probleme.
		});
	    },

(It's in a dialog function)