Issues with Plot tab after Nina Exo

Siril version: master

3 strange behaviours were observed:

• while the NINA Exo process is running:

  One can see the Y-scale choice toggle, depending on the previous kind of data you played with.

• at the end of the NINA Exo process:

  The ``Light Curve`` button is ON, whatever is the Y-scale data selected. Clicking on it leads to a new graph of the selected data. Even if this is an interresting feature, this is mostly an unwanted one, as the light curve should only be accessible while in Magnitude mode.

• in the previous condition and for a very strange reason, if the displayed data choice is ``background`` (in the drop-list), it leads to an application freeze (white screen in Siril Plot) and Siril get unresponsive.