gputop-ui per context view breaks when one of the contexts goes away

[eero-t]

Test-case:

1. Open "Live" counters" view from gputop-ui and "Start sampling"
2. Open "Per contexts" view and select multiple contexts from "Select contexts"
3. Add "Gpu Busy" and "AVG Gpu Core Fequency" counter timelines from "Live counters" view
4. Close program corresponding to one of the selected contexts

Expected outcome:

• When context disappears, timelines corresponding to that context disappear

Actual outcome:

• Timelines disappear for all contexts
• User needs to close the whole view, re-open it, reselect the contexts and add counters, to continue monitoring rest of the contexts

I think multicontext analysis is the main advantage of this tool over command line version of this tool and other open source GPU tools that work with Intel (intel_gpu_top in IGT, Apitrace and FrameRetrace), so it would be good to have that working well.