Bootstrap modal require is not defined for ArcGIS server map

Please forgive me if this is very simple but I am not be able to find out the work around to solve the issue.

I am trying to open ESRI map in a bootstrap modal. On the first time it doesn't load the map (in console I see the error "require is not defined") but second time it works fine. If I open the map in a separate window then it also work well each time.

My partial view looks like following:

@{
    <link rel="stylesheet" href="https://js.arcgis.com/3.21/dijit/themes/claro/claro.css">
    <link rel="stylesheet" href="https://js.arcgis.com/3.21/esri/css/esri.css">

    <script src="https://js.arcgis.com/3.21/"></script>
    <script>
        var map;

        require([
          "esri/map",
          "dojo/parser",
          "dojo/domReady!"
        ],
        function (
          Map,
          parser
        )
        {
            parser.parse();

            map = new Map("map", {
                basemap: "streets",
                center: [5.79, 50.97], // lon, lat
                zoom: 16,
                slider: false
            });
        });
   </script>
}

<div class="modal-header">
    <a class="close" data-dismiss="modal">&times;</a>
    <h4>DrawProjectLocation</h4>
</div>
<div class="modal-body">
    <div id="mainWindow" data-dojo-type="dijit/layout/BorderContainer"  data-dojo-props="design:'headline', gutters:false" style="width:100%; height:100%;">      
        <div id="map" class="roundedCorners" data-dojo-type="dijit/layout/ContentPane" data-dojo-props="region:'center'">

        </div>
        <div id="footer" class="roundedCorners" data-dojo-type="dijit/layout/ContentPane" data-dojo-props="region:'bottom'">
            <div id="editorDiv"></div>
        </div>
    </div>
</div>
<div class="modal-footer">
    <span id="info" style="position:absolute; left:15px; bottom:15px; color:#000; z-index:100"></span>
    <button type="submit" class="btn btn-success" id="submitButton">Save</button>
    <button type="button" class="btn btn-default" data-dismiss="modal">Cancel</button>
</div>

I tried to include require.js in my partial view but it didn't help. Can somebody please tell me whats going wrong here. Thanks!!