The problem has 2 folds.
The first one is that the shutdown function on DevToolsExtensionPageContextParent
wasn't called, which means the target was never destroyed. This is fixed by
overriding unload in DevToolsExtensionPageContextParent instead of shutdown, so
the function gets called and the target destroyed.
The second issue was that a single webextension could create 2 targets, and since
we only keep track of a single target, we would miss one. This is fixed by putting
the call to watchTargets in getCurrentDevTools in a promise, so subsequent calls
that might occur before the resulting promise isn't resolved don't end up calling
watchTargets a second time.
Differential Revision: https://phabricator.services.mozilla.com/D83935
ae24ed2cde