Code review comment for lp://staging/~mefrio-g/scratch/fix-1258260

It still doesn't work that well for me:

Open scratch (No tabs are open and the welcome screen is shown)

Open a Vala file
=> Outline pane has two Symbols tabs (the inactive tab isn't clickable)

Open a second Vala file
Switch between the tabs
=> The output pane no longer shows any content
=> For each switch, a new Symbols tab is added to the Outline pane

Close both tabs
=> failed assertion and SIGABRT when the last tab is closed

If I instead open Scratch, create a new document and then open a Vala document, it seems to work.
I can switch between the two tabs and the outline pane is shown or hidden accordingly. The outline pane also has the right content.