Fix missing inplace=True in validation of visualization tables (#214)

Closes #213

Author: dweindl

petab/visualize/lint.py

@@ -124,7 +124,7 @@ def set_default(column: str, value):
         if column not in vis_df:
             vis_df[column] = value
         elif value is not None:
-            vis_df[column].fillna(value)
+            vis_df[column].fillna(value, inplace=True)
 
     set_default(C.PLOT_NAME, "")
     set_default(C.PLOT_TYPE_SIMULATION, C.LINE_PLOT)