add-source scheduler crashes app because of unhandled exception

[sssoleileraaa]

Extracting #906 (comment) from the long list of issues in #906 since there is a fix and str now:

STR

1. Run NUM_SOURCES=50 make dev
2. Log into the client so that 50 source widgets are create
3. Close the client and run NUM_SOURCES=50 make dev
4. Log into the client again

Expected

For the old source widgets to disappear and the new ones to be added

Actual

The client crashes with the following traceback:

Traceback (most recent call last):
  File "/home/creviera/workspace/freedomofpress/securedrop-client/securedrop_client/gui/widgets.py", line 859, in schedule_source_management
    new_source = SourceWidget(self.controller, source)
  File "/home/creviera/workspace/freedomofpress/securedrop-client/securedrop_client/gui/widgets.py", line 1034, in __init__
    self.update()
  File "/home/creviera/workspace/freedomofpress/securedrop-client/securedrop_client/gui/widgets.py", line 1041, in update
    self.controller.session.refresh(self.source)
  File "/home/creviera/workspace/freedomofpress/securedrop-client/.venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 1675, in refresh
    "Could not refresh instance '%s'" % instance_str(instance)
sqlalchemy.exc.InvalidRequestError: Could not refresh instance '<Source at 0x7f5b46c37550>'

What's behind this?

We raise an exception in the SourceWidget.update method that we don't handle when the intial_update add-source scheduling happens, see:

securedrop-client/securedrop_client/gui/widgets.py

Line 1056 in aa89e70

raise

and

securedrop-client/securedrop_client/gui/widgets.py

Line 597 in aa89e70

self.source_list.initial_update(sources)