An internal error occurred during: "Loading descriptor for ... java.lang.NullPointerException

Question

When ever I refresh my Dynamic Web Project (or save a file), it gives met his error. It actually usually displays the same error three times.

The other answers I've looked at here suggest it only happens when the person creates a new project. This has started happening just recently.

Looking at the workspace/.metadata/.log shows me a range of errors:

!ENTRY org.eclipse.jst.j2ee 4 0 2015-05-15 15:20:16.141
!MESSAGE No IModelProvider exists for project P/MyWebProject of version: Dynamic Web Module 2.5
!STACK 0
java.lang.NullPointerException: No IModelProvider exists for project P/MyWebProject of version: Dynamic Web Module 2.5
    at org.eclipse.jst.j2ee.model.ModelProviderManager.getModelProvider(ModelProviderManager.java:101)
    at org.eclipse.jst.j2ee.model.ModelProviderManager.getModelProvider(ModelProviderManager.java:281)
    at org.eclipse.jst.jee.ui.internal.navigator.JEE5ContentProvider.getCachedModelProvider(JEE5ContentProvider.java:76)
    at org.eclipse.jst.jee.ui.internal.navigator.Web25ContentProvider.getNewContentProviderInstance(Web25ContentProvider.java:69)
    at org.eclipse.jst.jee.ui.internal.navigator.LoadingJeeDDJob.run(LoadingJeeDDJob.java:50)
    at org.eclipse.core.internal.jobs.Worker.run(Worker.java:53)

!ENTRY org.eclipse.core.jobs 4 2 2015-05-15 15:20:16.141
!MESSAGE An internal error occurred during: "Loading descriptor for MyWebProject.".
!STACK 0
java.lang.NullPointerException
    at org.eclipse.jst.jee.ui.internal.navigator.JEE5ContentProvider.getCachedModelProvider(JEE5ContentProvider.java:77)
    at org.eclipse.jst.jee.ui.internal.navigator.Web25ContentProvider.getNewContentProviderInstance(Web25ContentProvider.java:69)
    at org.eclipse.jst.jee.ui.internal.navigator.LoadingJeeDDJob.run(LoadingJeeDDJob.java:50)
    at org.eclipse.core.internal.jobs.Worker.run(Worker.java:53)

Answer 1

This problem had occurred in many forms, e.g.
Error when creating a new Vaadin project in eclipse

Solution: install these:

• Eclipse Java EE Developer Tools
• Eclipse Java Web Developer Tools
• JST Server Adapters
• JST Server Adapters Extensions

(In my case, Eclipse Java Web Developer Tools was missing.)