Ran into this error in eclipse today
eclipse An internal error occurred during "Initializing Java Tooling" the return value of "org.eclipse.jdt.internal.compiler.util.JRTUtil.getJrtSystem(java.io.File, String)" is null
After some google around, I found that the problem is a known issue, and is caused when a JDK does not include a "release" file in its root directory.
I had recently changed my development environment to use an unzipped version of JDK 1 and, after checking the directory, found that it was missing the file.
After downloading a more recent release of JDK11 from oracle.com, eclipse returned to normal
