Here is a try run where that happens:
https://treeherder.mozilla.org/#/jobs?repo=try&selectedJob=281794043&revision=485a9475618f8e5cdf096e074ed4b8aadd3b5c57

TEST-START | devtools/client/webconsole/test/browser/browser_console_clear_cache.js
[task 2019-12-18T17:39:59.536Z] 17:39:59     INFO - GECKO(2654) | console.log: "Request to connect to ProcessDescriptor \"2980\" failed: Error: Connection closed, pending request to server3.conn0.processDescriptor43, type getTarget failed\n\nRequest stack:\nrequest@resource://devtools/shared/protocol/Front.js:214:14\ngenerateRequestMethods/</frontProto[name]@resource://devtools/shared/protocol/Front/FrontClassWithSpec.js:49:19\ngetTarget/this._targetFrontPromise<@resource://devtools/shared/fronts/descriptors/process.js:74:40\ngetTarget@resource://devtools/shared/fronts/descriptors/process.js:91:7\n_processListChanged@resource://devtools/shared/resources/target-list.js:51:41\nAsync*emit@resource://devtools/shared/event-emitter.js:195:24\nemit@resource://devtools/shared/event-emitter.js:276:18\nonPacket@resource://devtools/shared/protocol/Front.js:252:13\nonPacket@resource://devtools/shared/client/debugger-client.js:496:13\nsend/<@resource://devtools/shared/transport/local-transport.js:70:25\nexports.makeInfallible/<@resource://devtools/shared/ThreadSafeDevToolsUtils.js:111:22\nDevToolsUtils.executeSoon*exports.executeSoon@resource://devtools/shared/DevToolsUtils.js:62:21\nsend@resource://devtools/shared/transport/local-transport.js:58:21\nsend@resource://devtools/server/debugger-server-connection.js:91:20\nonProcessListChanged@resource://devtools/server/actors/root.js:562:15\nobserve@resource://devtools/server/actors/process.js:73:12\n"
[task 2019-12-18T17:39:59.536Z] 17:39:59     INFO - GECKO(2654) | console.error: "Wasn't able to retrieve the target for" null
[task 2019-12-18T17:39:59.564Z] 17:39:59     INFO - GECKO(2654) | console.warn: "IGNORED REDUX ACTION:" ({type:"AUTOCOMPLETE_CLEAR"})
[task 2019-12-18T17:39:59.665Z] 17:39:59     INFO - GECKO(2654) | console.error: "Error while calling actor 'processDescriptor's method 'getTarget'" "this.transport is null"
[task 2019-12-18T17:39:59.666Z] 17:39:59     INFO - GECKO(2654) | console.error: "send@resource://devtools/server/debugger-server-connection.js:91:5\nsendReturn@resource://devtools/shared/protocol/Actor.js:180:16\npromise callback*generateRequestHandlers/</handler/<@resource://devtools/shared/protocol/Actor.js:186:14\n_queueResponse@resource://devtools/shared/protocol/Actor.js:107:28\nhandler@resource://devtools/shared/protocol/Actor.js:183:14\nonPacket@resource://devtools/server/debugger-server-connection.js:380:58\nsend/<@resource://devtools/shared/transport/local-transport.js:70:25\nexports.makeInfallible/<@resource://devtools/shared/ThreadSafeDevToolsUtils.js:111:22\nDevToolsUtils.executeSoon*exports.executeSoon@resource://devtools/shared/DevToolsUtils.js:62:21\nsend@resource://devtools/shared/transport/local-transport.js:58:21\nsend@resource://devtools/shared/protocol/Front.js:198:30\nrequest@resource://devtools/shared/protocol/Front.js:216:10\ngenerateRequestMethods/</frontProto[name]@resource://devtools/shared/protocol/Front/FrontClassWithSpec.js:49:19\ngetTarget/this._targetFrontPromise<@resource://devtools/shared/fronts/descriptors/process.js:74:40\ngetTarget@resource://devtools/shared/fronts/descriptors/process.js:91:7\n_processListChanged@resource://devtools/shared/resources/target-list.js:51:41\nAsync*emit@resource://devtools/shared/event-emitter.js:195:24\nemit@resource://devtools/shared/event-emitter.js:276:18\nonPacket@resource://devtools/shared/protocol/Front.js:252:13\nonPacket@resource://devtools/shared/client/debugger-client.js:496:13\nsend/<@resource://devtools/shared/transport/local-transport.js:70:25\nexports.makeInfallible/<@resource://devtools/shared/ThreadSafeDevToolsUtils.js:111:22\nDevToolsUtils.executeSoon*exports.executeSoon@resource://devtools/shared/DevToolsUtils.js:62:21\nsend@resource://devtools/shared/transport/local-transport.js:58:21\nsend@resource://devtools/server/debugger-server-connection.js:91:20\nonProcessListChanged@resource://devtools/server/actors/root.js:562:15\nobserve@resource://devtools/server/actors/process.js:73:12\n"
[task 2019-12-18T17:39:59.667Z] 17:39:59     INFO - GECKO(2654) | JavaScript error: resource://devtools/server/debugger-server-connection.js, line 91: TypeError: this.transport is null

Unfortunately, I haven't been able to reproduce it locally, which makes it hard to investigate.

Tracking DevTools tests for Fission Nightly (M6)

The priority flag is not set for this bug.
:nchevobbe, could you have a look please?

For more information, please visit auto_nag documentation.

Adding dt-fission whiteboard tag to DevTools bugs that mention Fission or block Fission meta bugs but don't already have a dt-fission whiteboard tag.

Moving these DevTools Fission bugs from Fission's old M6 Nightly milestone to M7 Beta. I am assuming these bugs would have the dt-fission-m2-mvp whiteboard tag if they were Fission Nightly blockers.

Bulk change of all bugs with whiteboard tag of dt-fission to Fission MVP milestone.

Moving old "dt-fission" bugs to "dt-fission-future" because they don't block Fission MVP.

I'm re-setting the pref to true in Bug 1642599 (or the test won't pass).
So let's see if we get intermittent (I ran those with --verify just fine)

old issue, let's close as we would get individual intermittent bugs for this