Crash while running mpmath chart tests.

[petersbingham]

Got the following stack trace. A one off; previous and subsequent tests on same code did not give this error:

Plot all data as cross section.
Plot center imag data as cross section. All parameters.
Plot center data as S-matrix, 1,1 element, imag and both axis logged.
Traceback (most recent call last):
  File ".\chart.py", line 66, in <module>
    i=0,j=0)
  File "E:\Peter's Documents\PhD\Code\Git\reskit\reskit\tools\chart\__init__.py", line 146, in plot_Smatrix
    self._plot(dmat, start, end, num_plot_points, logx, logy, imag, i, j)
  File "E:\Peter's Documents\PhD\Code\Git\reskit\reskit\tools\chart\__init__.py", line 95, in _plot
    add_axis_lmts=True)
  File "C:\Python27\lib\site-packages\matfuncutil\discrete.py", line 225, in plot
    plt.show()
  File "C:\Python27\lib\site-packages\matplotlib\pyplot.py", line 251, in show
    return _show(*args, **kw)
  File "C:\Python27\lib\site-packages\matplotlib\backend_bases.py", line 201, in show
    manager.show()
  File "C:\Python27\lib\site-packages\matplotlib\backends\backend_tkagg.py", line 562, in show
    self.canvas.manager.window.attributes('-topmost', 1)
AttributeError: 'NoneType' object has no attribute 'attributes'