Error appeared when I tried to edit attribute of <h:form>. It usually appears when I edit attributes. I can hardly reproduce this bug but it appears very often.
See attachment and error log below:

java.lang.NullPointerException
at org.eclipse.jst.jsp.core.internal.validation.JSPActionValidator.getTaglibPrefixes(JSPActionValidator.java:157)
at org.eclipse.jst.jsp.core.internal.validation.JSPActionValidator.performValidation(JSPActionValidator.java:215)
at org.eclipse.jst.jsp.core.internal.validation.JSPActionValidator.performValidation(JSPActionValidator.java:193)
at org.eclipse.jst.jsp.core.internal.validation.JSPBatchValidator.performValidation(JSPBatchValidator.java:377)
at org.eclipse.jst.jsp.core.internal.validation.JSPBatchValidator.validateFile(JSPBatchValidator.java:428)
at org.eclipse.jst.jsp.core.internal.validation.JSPBatchValidator.doValidate(JSPBatchValidator.java:190)
at org.eclipse.jst.jsp.core.internal.validation.JSPBatchValidator$1.run(JSPBatchValidator.java:452)
at org.eclipse.jdt.internal.core.BatchOperation.executeOperation(BatchOperation.java:39)
at org.eclipse.jdt.internal.core.JavaModelOperation.run(JavaModelOperation.java:720)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:1797)
at org.eclipse.jdt.core.JavaCore.run(JavaCore.java:4466)
at org.eclipse.jst.jsp.core.internal.validation.JSPBatchValidator.validateInJob(JSPBatchValidator.java:460)
at org.eclipse.wst.validation.internal.operations.ValidatorJob.run(ValidatorJob.java:75)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)