IllegalStateException("Disk Cache Not initialized") 怎样避免这个意外，因为这个意外出现的很频繁，谢谢了先

[autoool]

public Bitmap getBitmap(String url) {
try {
return mImageCache.getBitmap(createKey(url));
} catch (NullPointerException e) {
throw new IllegalStateException("Disk Cache Not initialized");
}
}