Fix show_plot due to bad return of ax.

Author: shyuep

src/pymatgen/analysis/diffraction/core.py

@@ -165,7 +165,7 @@ def show_plot(self, structure: Structure, **kwargs):
                 version (oriented vertically), e.g. 100. If 'full', show
                 long version, e.g. (1, 0, 0). If None, do not show anything.
         """
-        self.get_plot(structure, **kwargs).show()
+        self.get_plot(structure, **kwargs).get_figure().show()
 
     @add_fig_kwargs
     def plot_structures(self, structures, fontsize=6, **kwargs):