Steps to reproduce the issue

• Install nightly build Joomla_4.0.0-alpha5-dev-Development-Full_Package.zip
• Activate debug (Global configuration)
• Go into Plugins > "System - Debug"
• Check first that plugin is enabled
• Set "Refresh Assets" : NO
• Save plugin

Expected result

• Plugin saved and displays the settings afterwards.

Actual result

• Plugin saved but no settings visible afterwards
• No message: "Plugin saved"
• Button Saveand button Closewithout function
• Javascript error:

TypeError: script.src.match(...) is null core.js:1102:26
Joomla.WebComponents
http://.../media/system/js/core.js:1102:26

• I don't know if related: Additional warning with FF 62.0 64bit (but not with FF 52.9 ESR 32bit)

Source Map Error: request failed with status 404
Ressource Address: http://.../media/vendor/bootstrap/js/bootstrap.bundle.js
Source-Map-Address: bootstrap.bundle.js.map 

• Clearing browser cache doesn't help

System information (as much as possible)

PHP Built On Linux dd1620 3.13.0-157-generic #207-Ubuntu SMP Mon Aug 20 16:44:59 UTC 2018 x86_64
Database Type mysql
Database Version 5.5.58-nmm1-log
Database Collation utf8_general_ci
Database Connection Collation utf8_general_ci
PHP Version 7.2.7
Web Server Apache
WebServer to PHP Interface fpm-fcgi
Joomla! Version Joomla! 4.0.0-alpha5-dev Development [ Amani ] 22-July-2018 22:58 GMT

User Agent Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:62.0) Gecko/20100101 Firefox/62.0
AND
Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Firefox/52.9.0 ESR