Missing module information causes compiler error on eclipse.

[mabiste]

After checking out the validator project on my Eclipse IDE, I've got some compiler errors (such as 'AttributesImpl cannot be resolved to a type').

While searching for a solution, I've found the following stack overflow questions discussing a similar issue:
Eclipse can't find XML related classes after switching build path to JDK 10

I've solved the issue by adding the property
org.eclipse.jdt.core.compiler.ignoreUnnamedModuleForSplitPackage=enabled
to
.settings/org.eclipse.jdt.core.prefs file
as descriped on this Answer of the previously linked stack overflow question.