Prevent User Clicking UI When Using Async/Wait

I have a Task that returns an ObservableCollection of Companies. I display a small Window that simply says "Please wait.." whilst this Task is performed. The problem is, the user can still interact with the UI when this Task is running. I need to lock down the UI until this has completed.

Here is the code I have used so far;

private async void FillDataGrid()
{var Companies = new ObservableCollection<CompanyModel>();
        var waitWindow = new PleaseWait();

        waitWindow.Show();
        Companies = await ReturnCompanies();
        CompanyICollectionView = CollectionViewSource.GetDefaultView(Companies);
        DataContext = this;
        waitWindow.Close();
}

private Task<ObservableCollection<CompanyModel>> ReturnCompanies()
{
    try
    {
        var _cDS = new CompanyDataService();
        return Task.Run(() =>
        {
            return _cDS.HandleCompanySelect();
        });
    }
    catch (Exception ex)
    {
        var hEs = new HandleExceptionService();
        hEs.HandleException(ex.ToString());
        return null;
    }
}