sometimes psiFile.getVirtualFile() returns null in my referenceContributor

My referenceContributor extends from PsiReferenceContributor and is registered for XmlPatterns.xmlAttributeValue().

In its 

new PsiReferenceProvider() {
    @NotNull
    @Override
    public PsiReference[] getReferencesByElement(@NotNull PsiElement element,
                                                 @NotNull ProcessingContext context) {

I get the VirtualFile by using element.getContainingFile().getVirtualFile(), I need it for checking the file path.

I add a break point on that line, when user opens the XML file the break point get triggered, it works fine. But later when user click Ctrl+Space on any XML attribute value, it gets triggered again, this time the virtual file is null.

I don't understand why the referenceContributor get triggered, isn't the CompletionContributor for handling Ctrl+Space? (I tested, actually both get triggered.)

And why the virtual file is null, document says it's an in-memory file? But the XML file is stored in disk.

Thanks