(Another) GWT exception in Development Mode

Lately I got a new kind of exception during GWT Development Mode startup.

java.lang.NullPointerException
 at com.google.gwt.dev.resource.impl.DirectoryClassPathEntry.descendToFindResources(DirectoryClassPathEntry.java:79)
 at com.google.gwt.dev.resource.impl.DirectoryClassPathEntry.findApplicableResources(DirectoryClassPathEntry.java:54)
 at com.google.gwt.dev.resource.impl.ResourceOracleImpl.refresh(ResourceOracleImpl.java:351)
 at com.google.gwt.dev.cfg.ModuleDef.refresh(ModuleDef.java:386)
 at com.google.gwt.dev.cfg.ModuleDefLoader.tryGetLoadedModule(ModuleDefLoader.java:150)
 at 
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