def _getUniqueHistogram(plt):
    """Return the histogram if there is a single histogram and no curve in
    the plot. In all other cases, return None.

    :param plt: :class:`.PlotWidget` instance on which to operate
    :return: histogram or None
    """
    histograms = plt._getItems(kind='histogram')
    if len(histograms) != 1:
        return None
    if plt.getAllCurves(just_legend=True):
        return None
    return histograms[0]