Error when clicking the refresh button after animation

Problem description

When making an animation and clicking the refresh button, one gets the following error (see Screencast below)

Traceback (most recent call last):
File "/home/psommer/Documents/code/development/psyplot/psy-view/psy_view/ds_widget.py", line 333, in reload
num = next(num for num, f in sp.dsnames_map.items() if f == fname)
StopIteration

https://user-images.githubusercontent.com/9960249/116358737-41a81b80-a7fe-11eb-8487-5993080e97e3.mp4

Output of psyplot -aV

``` psy_maps.plugin: requirements: cartopy: 0.18.0 version: 1.3.2+2.g26b6411.dirty psy_reg.plugin: requirements: scipy: 1.5.3 statsmodels: 0.12.2 version: 1.3.0+3.gc849e50 psy_simple.plugin: version: 1.3.1 psy_strat.plugin: version: 0.1.0 psy_transect.plugin: version: 0.0.1.dev0 psyplot: requirements: matplotlib: 3.4.1 numpy: 1.20.2 pandas: 1.2.3 python: 3.8.6 | packaged by conda-forge | (default, Oct 7 2020, 19:08:05) [GCC 7.5.0] xarray: 0.17.0 version: 1.3.1 psyplot_gui: requirements: pyqt: 5.12.3 qt: 5.12.9 qtconsole: 5.0.3 version: 1.3.0

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