Application crash while accessing data element of linked list when data is NULL - ios

Question

I am using following code to traverse through linked list in objective c
const MSList *calls = linphone_core_get_calls(LC);
if (calls == NULL)
{
[self dismissCtrl];
//how to check current is which screen is on
// while ((currentView == CallView.compositeViewDescription) ||
// (currentView == CallIncomingView.compositeViewDescription) ||
// (currentView == CallOutgoingView.compositeViewDescription)) {
// [self popCurrentView];
// }
} else {
linphone_call_resume((LinphoneCall *)calls->data);
while (calls)
{
if(calls->data != NULL && calls->data != nil && calls->data != (__bridge void *)((id)[NSNull null]))
{
//crash
if (linphone_call_get_state((LinphoneCall *)calls->data) == LinphoneCallIncomingReceived ||
linphone_call_get_state((LinphoneCall *)calls->data) == LinphoneCallIncomingEarlyMedia) {
[self displayIncomingCall:(LinphoneCall *)calls->data];
break;
}
}
calls = calls->next;
}
Application is crashing when entire list is not null but its data,previous or next value is NULL. I have added code to check if data is NULL but if it(data) is NULL, then i will not be able to access it and in condition itself application is crashing. How to prevent this ? I have attached screen-shot for where application is crashing and what is the value that list contains at that time.

Answer 1

if (calls == NULL) {
[self dismissCtrl];
//how to check current is which screen is on
// while ((currentView == CallView.compositeViewDescription) ||
// (currentView == CallIncomingView.compositeViewDescription) ||
// (currentView == CallOutgoingView.compositeViewDescription)) {
// [self popCurrentView];
// }
}
else
{
size_t count = bctbx_list_size(calls);
linphone_call_resume((LinphoneCall *)calls->data);
int i = 0;
while (calls )
{
if(i < count)
{
if ( calls->data == (__bridge void *)((id)[NSNull null]) || calls->data == NULL || calls->data == nil )
{
return;
}
LinphoneCall *objCall = (LinphoneCall *)calls->data;
LinphoneCallState state = (objCall != NULL) ? linphone_call_get_state(call) : 0;
if (state == LinphoneCallIncomingReceived ||
state == LinphoneCallIncomingEarlyMedia)
{
[self displayIncomingCall:(LinphoneCall *)calls->data];
break;
}
//calls ? calls->data : NULL
// calls = calls ? calls->next : NULL;
calls = calls->next;
i = i+1;
}
else
{
break;
}
}