Note: this all applies when installing a WebExtension theme in about:debugging. Apart from the fact that the addon shows up in the Addon Manager, saying 'Will be enabled after a restart', the following error appears in the console when enabling/ disabling it there: 1489510724266 addons.manager WARN AddonListener threw exception when calling onEnabling: TypeError: themeFor(...) is null (resource://gre/modules/LightweightThemeManager.jsm:460:12) JS Stack trace: get id@LightweightThemeManager.jsm:460:12 < delegateAddonEvent@extensions.js:619:9 < initialize/this[event]@extensions.js:514:35 < callAddonListeners@AddonManager.jsm:1693:11 < callAddonListeners@AddonManager.jsm:3160:5 < addonChanged@LightweightThemeManager.jsm:397:9 < callProvider@AddonManager.jsm:268:12 < notifyAddonChanged@AddonManager.jsm:1741:7 < notifyAddonChanged@AddonManager.jsm:3141:5 < updateAddonDisabledState@XPIProvider.jsm:5112:7 < set userDisabled@XPIProvider.jsm:7535:9 < doCommand@extensions.js:1303:9 < set_userDisabled@extensions.xml:1108:13 < oncommand@about:addons:1:1 1489510724268 addons.manager WARN AddonListener threw exception when calling onEnabled: TypeError: themeFor(...) is null (resource://gre/modules/LightweightThemeManager.jsm:460:12) JS Stack trace: get id@LightweightThemeManager.jsm:460:12 < delegateAddonEvent@extensions.js:619:9 < initialize/this[event]@extensions.js:514:35 < callAddonListeners@AddonManager.jsm:1693:11 < callAddonListeners@AddonManager.jsm:3160:5 < addonChanged@LightweightThemeManager.jsm:399:9 < callProvider@AddonManager.jsm:268:12 < notifyAddonChanged@AddonManager.jsm:1741:7 < notifyAddonChanged@AddonManager.jsm:3141:5 < updateAddonDisabledState@XPIProvider.jsm:5112:7 < set userDisabled@XPIProvider.jsm:7535:9 < doCommand@extensions.js:1303:9 < set_userDisabled@extensions.xml:1108:13 < oncommand@about:addons:1:1

Comment on attachment 8854877 [details] Bug 1347226 - prevent an error thrown by LightWeightThemeManager when a temporary WebExtension theme addon is installed and make sure it's enabled correctly in the Addon Manager. https://reviewboard.mozilla.org/r/126822/#review129970

Pushed by mdeboer@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/f8f2cb57530f prevent an error thrown by LightWeightThemeManager when a temporary WebExtension theme addon is installed and make sure it's enabled correctly in the Addon Manager. r=mossop