I got the following exception when configuring content assist (adding the Java completion proposals which were missing for me):

java.lang.NullPointerException
at org.jboss.tools.common.ui.preferences.SeverityPreferencePage.updateEnableBlock(SeverityPreferencePage.java:155)
at org.jboss.tools.common.ui.preferences.SeverityPreferencePage.performOk(SeverityPreferencePage.java:138)
at org.eclipse.jface.preference.PreferenceDialog$13.run(PreferenceDialog.java:964)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.ui.internal.JFaceUtil$1.run(JFaceUtil.java:49)
at org.eclipse.jface.util.SafeRunnable.run(SafeRunnable.java:175)
at org.eclipse.jface.preference.PreferenceDialog.okPressed(PreferenceDialog.java:944)
at org.eclipse.ui.internal.dialogs.FilteredPreferenceDialog.okPressed(FilteredPreferenceDialog.java:447)
at org.eclipse.ui.internal.dialogs.WorkbenchPreferenceDialog.okPressed(WorkbenchPreferenceDialog.java:171)
at org.eclipse.jface.preference.PreferenceDialog.buttonPressed(PreferenceDialog.java:233)
at org.eclipse.jface.dialogs.Dialog$2.widgetSelected(Dialog.java:624)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:240)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4128)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1457)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1480)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1465)
at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:1270)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3974)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3613)
at org.eclipse.jface.window.Window.runEventLoop(Window.java:825)
at org.eclipse.jface.window.Window.open(Window.java:801)
at org.eclipse.ui.internal.dialogs.WorkbenchPreferenceDialog.open(WorkbenchPreferenceDialog.java:215)
at org.eclipse.ui.internal.handlers.ShowPreferencePageHandler.execute(ShowPreferencePageHandler.java:54)
at org.eclipse.ui.internal.handlers.HandlerProxy.execute(HandlerProxy.java:293)
at org.eclipse.core.commands.Command.executeWithChecks(Command.java:476)
at org.eclipse.ui.internal.handlers.HandlerService.executeCommand(HandlerService.java:178)
at org.eclipse.ui.internal.cocoa.CocoaUIEnhancer.runCommand(CocoaUIEnhancer.java:393)
at org.eclipse.ui.internal.cocoa.CocoaUIEnhancer.actionProc(CocoaUIEnhancer.java:535)
at org.eclipse.ui.internal.cocoa.CocoaUIEnhancer.actionProc(CocoaUIEnhancer.java:520)
at org.eclipse.swt.internal.cocoa.OS.objc_msgSendSuper(Native Method)
at org.eclipse.swt.widgets.Display.applicationNextEventMatchingMask(Display.java:4879)
at org.eclipse.swt.widgets.Display.applicationProc(Display.java:5226)
at org.eclipse.swt.internal.cocoa.OS.objc_msgSend(Native Method)
at org.eclipse.swt.internal.cocoa.NSApplication.nextEventMatchingMask(NSApplication.java:94)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3607)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2701)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2665)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2499)
at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:679)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:668)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:123)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:344)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:622)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:577)
at org.eclipse.equinox.launcher.Main.run(Main.java:1410)