fix some issues revealed by tests and github doc build

Author: iboyd-ansys

src/ansys/systemcoupling/core/charts/plotdefinition_manager.py

@@ -243,6 +243,10 @@ def set_metadata(self, metadata: InterfaceInfo):
                 if transfer.transfer_display_name not in active_transfers:
                     # We don't want this transfer on the convergence plot
                     continue
+                if self._conv_subplot is None:
+                    # This will be the case if the plot spec had `show_convergence=False`
+                    continue
+
                 # Clear the entry in case transfer names are not unique. (If another transfer
                 # with the same name needs plotting, then it should appear as a second entry
                 # in active_transfers.)

src/ansys/systemcoupling/core/charts/plotter.py

@@ -396,7 +396,7 @@ def update_line_series(self, series_data: SeriesData):
 
     def close(self):
         for fig in self._figures:
-            plt.close(fig)
+            fig.close()
 
     def show_plot(self, noblock=False):
         if noblock: