grid does not support add_rows on an empty dataframe. Could we stop the user from removing the last row?

[camilog04]

Environment

• Python 3.8.8
• pip installed

Description of Issue

• What did you expect to happen?
  when the dataframe is empty, the add row botton from the tool bar does not work.
• What happened instead?
  I expected that the widget add a row

Reproduction Steps

1. initialize the qgrid widget with one row
2. show the widget with the tool bar option
3. delete all the rows
4. click the 'add_row' button
   ...

What steps have you taken to resolve this already?

A solution to this is to prevent the users from deleting the rows. This can be accomplished by adding an empty check to the remove_rows method:

• if self._df.shape[0] <= 1:
  

•     # Optionally throw a warning here
  

•     return selected_names
  

...

Anything else?

Another solution is that when the dataframe is empty there is a constroctor of a 'default' row but but this would be more difficult to scale.
...