Getting error: "An internal build error has occurred... " when compiling

I am getting this error after upgrading to Flash Builder 4.5 from Flex 3. This error was not occurring under Flex 3.

I do not have an empty Case statement. If I remove a particular module from the Flex Modules list in the project propertues, the error does not occur, but of course no swf is generated so the module cannot be loaded at run time.
I have gone through the module in question with a fine toothed comb but cannot find any syntax errors

An internal build error has occurred. See the error log for more information.    Culinaria        Unknown    Flex Problem

null
Error
Mon Jun 20 10:41:22 CEST 2011
Uncaught exception in compiler

!ENTRY com.adobe.flexbuilder.project 4 43 2011-06-20 10:41:22.623
!MESSAGE Uncaught exception in compiler
!STACK 0
java.lang.NullPointerException
    at flex2.tools.PreLink.lookupStyleName(PreLink.java:719)
    at flex2.tools.PreLink.lookupBackgroundColor(PreLink.java:518)
    at flex2.tools.PreLink.postRun(PreLink.java:181)
    at flex2.compiler.CompilerAPI.compile(CompilerAPI.java:1592)
    at flex2.tools.oem.Application.compile(Application.java:1349)
    at flex2.tools.oem.Application.recompile(Application.java:1287)
    at flex2.tools.oem.Application.compile(Application.java:886)
    at flex2.tools.flexbuilder.BuilderApplication.compile(BuilderApplication .java:359)
    at com.adobe.flexbuilder.multisdk.compiler.internal.ASApplicationBuilder $MyBuilder.mybuild(ASApplicationBuilder.java:319)
    at com.adobe.flexbuilder.multisdk.compiler.internal.ModuleBuilder.build( ModuleBuilder.java:137)
    at com.adobe.flexbuilder.multisdk.compiler.internal.ASBuilder.build(ASBu ilder.java:198)
    at com.adobe.flexbuilder.multisdk.compiler.internal.ASItemBuilder.build( ASItemBuilder.java:70)
    at com.adobe.flexbuilder.project.compiler.internal.FlexProjectBuilder.bu ildItem(FlexProjectBuilder.java:575)
    at com.adobe.flexbuilder.project.compiler.internal.FlexProjectBuilder.bu ildModules(FlexProjectBuilder.java:474)
    at com.adobe.flexbuilder.project.compiler.internal.FlexProjectBuilder.bu ild(FlexProjectBuilder.java:363)
    at com.adobe.flexbuilder.project.compiler.internal.FlexIncrementalBuilde r.build(FlexIncrementalBuilder.java:187)
    at org.eclipse.core.internal.events.BuildManager$2.run(BuildManager.java :629)
    at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
    at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager .java:172)
    at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager .java:203)
    at org.eclipse.core.internal.events.BuildManager$1.run(BuildManager.java :255)
    at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
    at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager .java:258)
    at org.eclipse.core.internal.events.BuildManager.basicBuildLoop(BuildMan ager.java:311)
    at org.eclipse.core.internal.events.BuildManager.build(BuildManager.java :343)
    at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBuildJob.ja va:144)
    at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJob.java:2 42)
    at org.eclipse.core.internal.jobs.Worker.run(Worker.java:54)