Please look at this example. It works fine.
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Basic Tabs - jQuery EasyUI Demo</title>
<link rel="stylesheet" type="text/css" href="https://www.jeasyui.com/easyui/themes/default/easyui.css">
<link rel="stylesheet" type="text/css" href="https://www.jeasyui.com/easyui/themes/icon.css">
<script type="text/javascript" src="https://www.jeasyui.com/easyui/jquery.min.js"></script>
<script type="text/javascript" src="https://www.jeasyui.com/easyui/jquery.easyui.min.js"></script>
<script type="text/javascript">
$(function(){
$('#wu-tabs').tabs({
onBeforeClose: function(title){
$('#windowPopUp-content').appendTo('#windowPopUp');
$('#windowPopUp').window('open').window('resize');
}
})
$('#windowPopUp').window({
onMinimize: function(){
var title = 'მომხმარებლის შექმნა';
var tabPanel = $('#wu-tabs');
tabPanel.tabs('add', {
title: title,
content: '<div id="tabs-window" style="position:relative;width:100%;height:100%;overflow:auto"></div>',
fit: true,
closable: true
})
$('#windowPopUp-content').appendTo('#tabs-window');
$('#wu-tabs').tabs('resize')
}
})
})
var data = {"total":28,"rows":[
{"productid":"FI-SW-01","productname":"Koi","unitcost":"10.00","status":"P","listprice":"36.50","attr1":"Large","itemid":"EST-1"},
{"productid":"K9-DL-01","productname":"Dalmation","unitcost":"12.00","status":"P","listprice":"18.50","attr1":"Spotted Adult Female","itemid":"EST-10"},
{"productid":"RP-SN-01","productname":"Rattlesnake","unitcost":"12.00","status":"P","listprice":"38.50","attr1":"Venomless","itemid":"EST-11"},
{"productid":"RP-SN-01","productname":"Rattlesnake","unitcost":"12.00","status":"P","listprice":"26.50","attr1":"Rattleless","itemid":"EST-12"},
{"productid":"RP-LI-02","productname":"Iguana","unitcost":"12.00","status":"P","listprice":"35.50","attr1":"Green Adult","itemid":"EST-13"},
{"productid":"FL-DSH-01","productname":"Manx","unitcost":"12.00","status":"P","listprice":"158.50","attr1":"Tailless","itemid":"EST-14"},
{"productid":"FL-DSH-01","productname":"Manx","unitcost":"12.00","status":"P","listprice":"83.50","attr1":"With tail","itemid":"EST-15"},
{"productid":"FL-DLH-02","productname":"Persian","unitcost":"12.00","status":"P","listprice":"23.50","attr1":"Adult Female","itemid":"EST-16"},
{"productid":"FL-DLH-02","productname":"Persian","unitcost":"12.00","status":"P","listprice":"89.50","attr1":"Adult Male","itemid":"EST-17"},
{"productid":"AV-CB-01","productname":"Amazon Parrot","unitcost":"92.00","status":"P","listprice":"63.50","attr1":"Adult Male","itemid":"EST-18"}
]}
</script>
</head>
<body>
<div id="wu-tabs" style="width:700px;height:400px">
<div title="Base" style="padding:10px">
<p>Base Content.</p>
</div>
</div>
<div id="windowPopUp" title="The Title" style="width:700px;height:400px">
<div id="windowPopUp-content" style="width:100%;height:100%">
<table class="easyui-datagrid" title="Basic DataGrid"
data-options="singleSelect:true,collapsible:true,data:data,fit:true,border:false">
<thead>
<tr>
<th data-options="field:'itemid',width:80">Item ID</th>
<th data-options="field:'productid',width:100">Product</th>
<th data-options="field:'listprice',width:80,align:'right'">List Price</th>
<th data-options="field:'unitcost',width:80,align:'right'">Unit Cost</th>
<th data-options="field:'attr1',width:250">Attribute</th>
<th data-options="field:'status',width:60,align:'center'">Status</th>
</tr>
</thead>
</table>
</div>
</div>
</body>
</html>