So, I saw that Java has just been updated to java 9, but the launcher supports only version 8. Is there anything that is being developed to support JRE 9?
Just did some testing and research. This issue is caused by LaunchWrapper which is required for forge and the issue will not be fixed any time soon. So you will have to stick to java 8, you can have both java versions installed and put the java 8 path in the launcher settings. Details can be found here: Java 9 support for 1.12+ · Issue #3935 · MinecraftForge/MinecraftForge · GitHub