You should call IsValid method on any annot before you call other methods on it. For example, a FreeText annot does not have to have a popup, so calling
free_text.GetPopUp().GetContent()
is dangerous.
instead call
popup = free_text.GetPopup()
if (popup.IsValid()) {
popup.GetContent();
}
Of course a SIGSEGV is bad, and we recently fixed an issue with this regards, but the fix is not yet available for mobile yet, so you need to use the logic above.
If this does not help, that is you get a crash after IsValid returns true, or something similar, please file a bug report, including the code you are calling, and ideally the PDF file for which the issue happens with.
https://www.pdftron.com/support/reportproblem.html