DevTools test sometimes fails on Mac with TypeError: docShell.chromeEventHandler is null

[mykmelez]

The DevTools test (run-devtools.js) sometimes fails on Mac with TypeError: docShell.chromeEventHandler is null:

A coding exception was thrown and uncaught in a Task.
Full message: TypeError: docShell.chromeEventHandler is null
Full stack: Helpers_onLocationChange@resource://devtools/client/shared/DOMHelpers.jsm:163:7
open/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/client/framework/toolbox.js:408:7
_run@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/task.js:311:39
TaskImpl@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/task.js:273:3
asyncFunction@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/task.js:247:14
spawn@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/task.js:161:12
open@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/client/framework/toolbox.js:395:12
DevTools.prototype.createToolbox<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/client/framework/devtools.js:485:11
_run@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/task.js:311:39
process@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:922:23
walkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:806:7
Promise*scheduleWalkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:739:11
schedulePromise@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:770:7
completePromise@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:707:7
promise callback*completePromise@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:698:7
_run@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/task.js:316:13
resolve@resource://devtools/shared/deprecated-sync-thenables.js:42:40
then@resource://devtools/shared/deprecated-sync-thenables.js:22:43
resolve@resource://devtools/shared/deprecated-sync-thenables.js:74:11
listenerJson@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:734:9
emitOnObject@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:110:7
emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:86:38
emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:1323:29
emitReply@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:1022:29
DevTools RDP*request@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:718:21
getWindow@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:1757:12
exports.targetFromURL<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/client/framework/target-from-url.js:107:28
_run@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/task.js:311:39
resolve@resource://devtools/shared/deprecated-sync-thenables.js:42:40
then@resource://devtools/shared/deprecated-sync-thenables.js:22:43
resolve@resource://devtools/shared/deprecated-sync-thenables.js:74:11
connect/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:344:7
l@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:68:7
eventSource/proto.emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:130:9
exports.DebuggerClient/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:227:5
emitOnObject@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:110:7
emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/commonjs/sdk/event/core.js:86:38
emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:1323:29
emitReply@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:1022:29
onPacket@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/client/main.js:1027:9
send/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/transport/transport.js:570:13
exports.makeInfallible/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/ThreadSafeDevToolsUtils.js:101:14
exports.makeInfallible/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/ThreadSafeDevToolsUtils.js:101:14

For example, during this nightly test run: https://travis-ci.org/mozilla/qbrt/jobs/240062731. In that case, the test failed when testing DevTools for the XUL window target (i.e. chrome://app/content/window.xul), although I'm not sure that it always fails on that target.