buttercup-browser-extension: Source type is empty

At first I didn’t see any menu icons. All I saw was just the black background. After that I removed Buttercup from firefox, cleared the Cache and Cookies, restarted Firefox, installed Buttercup and tried it again. Now I see nothing:

System Info: Browser: Firefox 62.0 (64 Bit) System: MacOS 10.13.6 Webconsole:

Could not map contract ID '@mozilla.org/fxaccounts/push;1' to CID {1b7db999-2ecd-4abf-bb95-a726896798ca} because no implementation of the CID is registered. components.manifest:163
Laden fehlgeschlagen für das <script> mit der Quelle "moz-extension://30cf1a2e-cfdb-a749-a913-b18b3db4c0ad/background-buttercup.js". _generated_background_page.html:5
Laden fehlgeschlagen für das <script> mit der Quelle "moz-extension://30cf1a2e-cfdb-a749-a913-b18b3db4c0ad/background-vendor.js". _generated_background_page.html:6
Laden fehlgeschlagen für das <script> mit der Quelle "moz-extension://30cf1a2e-cfdb-a749-a913-b18b3db4c0ad/background-index.js". _generated_background_page.html:7
NS_ERROR_NOT_AVAILABLE: Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIWebNavigation.loadURIWithOptions] browser-child.js:352
Use of nsIFile in content process is deprecated. NetUtil.jsm:266:12
NS_ERROR_NOT_AVAILABLE: Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIWebNavigation.loadURIWithOptions] browser-child.js:352