Edit in JSFiddle

var flexGrid = new wijmo.grid.FlexGrid('#flexGrid');
flexGrid.itemsSource = createData(10);

$('#myModal').on('shown.bs.modal', function (e) {
    flexGrid.refresh();
});

function createData(count) {
    var countries = 'US,Germany,UK,Japan,Italy,Greece'.split(','),
        data = [];

    for (var i = 0; i < count; i++) {
        data.push({
            id: i + 1,
            country: countries[i % countries.length],
            date: new Date(2014, i % 12, i % 28),
            amount: Math.random() * 10000,
            active: i % 4 == 0
        });
    }

    return data;
}

<button type="button" class="btn btn-default" data-toggle="modal" data-target="#myModal">Show modal</button>

<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-body" style="height: 250px; overflow-y: auto;">
        <div id="flexGrid"  style="width:400px"></div>
      </div>
    </div>
  </div>
</div>