docs: @CheckForNull
      protected ICompilationUnit getCompilationUnit(IMarker marker) {
          IResource res = marker.getResource();
          if (res instanceof IFile && res.isAccessible()) {
              IJavaElement element = JavaCore.create((IFile) res);
              if (element instanceof ICompilationUnit) {
                  return (ICompilationUnit) element;
              }
          }
          return null;
      }