Update TextInput start/end entries from BoxSelectTool selection

def callback_box_select(attr, old, new):
    '''Update TextInput start/end entries from BoxSelectTool selection'''

    # Get indices of selection
    ind = sorted(new['1d']['indices'])

    if new is None:
        start_input.value = '0'
        end_input.value = '0'
    else:
        start_input.value = str(source.data['ind'][ind[0]])
        end_input.value = str(source.data['ind'][ind[-1]])
        msg = '''
              New start and end index values set.
              '''
        output_window.text = output_template.format(msg)

    return None