I think the cause of this might be if you try to go beyond the start or end of the breadcrumbs using the arrow keys, or by pressing a key that isn't an arrow key. A quick look at the code seems to agree with the latter. I also get this error on the console (some irrelevant bits removed): > A promise chain failed to handle a rejection. Did you forget to '.catch', or did you forget to 'return'? > See https://developer.mozilla.org/Mozilla/JavaScript_code_modules/Promise.jsm/Promise> > Date: Wed Aug 26 2015 20:25:23 GMT+1200 (NZST) > Full Message: TypeError: node is null > Full Stack: HTMLBreadcrumbs.prototype.handleKeyPress/this._keyPromise<@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/breadcrumbs.js:383:7 > ... > HTMLBreadcrumbs.prototype.handleKeyPress@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/breadcrumbs.js:359:25 > HTMLBreadcrumbs.prototype.handleEvent@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/breadcrumbs.js:288:7 > EventListener.handleEvent*HTMLBreadcrumbs.prototype._init@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/breadcrumbs.js:72:5 > HTMLBreadcrumbs@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/breadcrumbs.js:48:3 > InspectorPanel.prototype._deferredOpen@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/inspector-panel.js:158:24 > InspectorPanel.prototype.open/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/inspector-panel.js:98:14 > ... > Front<.onPacket/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/protocol.js:1228:9 > DevTools RDP*Front<.request@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/protocol.js:1178:7 > frontProto/</proto[name]@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/protocol.js:1330:14 > exports.WalkerFront<.querySelector<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/actors/inspector.js:3220:12 > InspectorPanel.prototype._getDefaultNodeForSelection/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/inspector-panel.js:265:14 > ... > Front<.onPacket/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/protocol.js:1228:9 > DevTools RDP*Front<.request@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/protocol.js:1178:7 > frontProto/</proto[name]@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/protocol.js:1330:14 > exports.InspectorFront<.getPageStyle<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/actors/inspector.js:3715:12 > InspectorPanel.prototype._getPageStyle@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/inspector-panel.js:226:12 > InspectorPanel.prototype.open/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/inspector-panel.js:94:14 > ... > InspectorPanel.prototype.open@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/inspector-panel.js:93:12 > Toolbox.prototype.loadTool/onLoad@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/framework/toolbox.js:1171:19

The node variable at breadcrumbs.js:383 can be null in some cases, so it should be a simple fix. However, I realized there are absolutely no tests covering this in the tree, adding one is what's going to take the most time here.

Comment on attachment 8652821 [details] [diff] [review] Bug_1198627_-_Prevent_errors_when_keyboard_navigat.diff Review of attachment 8652821 [details] [diff] [review]: ----------------------------------------------------------------- Nice!

While we're here, what happens to keyboard events that aren't from arrow keys? F12, for example. They don't seem to bubble up to a listener that does anything with them.

(In reply to Geoff Lankow (:darktrojan) from comment #5) > While we're here, what happens to keyboard events that aren't from arrow > keys? F12, for example. They don't seem to bubble up to a listener that does > anything with them. Let's deal with this in another bug. I filed bug 1199102 for this. Could you provide a bit more information in that bug?