可能是出现了某行为空

Let's check what ViewHandlerImpl#getActionURL() does (Mojarra is just open source). Here's is a summarized/stripped extract of the method as it is in Mojarra 2.0.2:

public String getActionURL(FacesContext context, String viewId) {
    if (context == null) throw new NullPointerException();
    if (viewId == null) throw new NullPointerException();
    if (viewId.charAt(0) != '/') throw new IllegalArgumentException();
    // ...

In other words, the passed-in viewId is an empty string instead of null or / or the normal path. The Mojarra viewhandler didn't expect it at all, hence this runtime exception. It can be a bug in the view handler of Mojarra that it should handle empty strings as well, but it can also be a bug in the view handler of either Ajax4jsf, Facelets or Seam that it should never pass an empty string through up. It can also be a bug in your own code that you're actually passing an empty string as view ID.

If the problem isn't in your code, then I would start with upgrading what can be upgraded to see if it resolves the problem. Maybe it was already reported before as a bug and fixed in a newer release. If that doesn't help, I would check the classpath for duplicate JAR files of different versions which might have collided with each other.