NPE when using footer as page loading indicator

[lianghanzhen]

I use ListView footer to indicate page loading, when loading next page, I scroll up and down again and again, after adapter notify, it throws NPE. you can see the issue in my fork SimpleListWithHeadersAndFootersActivity

[jbarr21]

I loaded your fork on Android 5.0, but saw no NPE. What version of Android do you see the issue on and with what effect?