Doubly Linked list, insert_after issue - c++

Question

I'm having trouble with an assignment of mine. I've been going over the code for over a day, yet still can't seem to fix the issue. I've tried gdb, identified where the problem was and everything, so i'd really appreciate some fresh, more experienced eyes on this.
I don't know if I need to include the entire program so i'll just start with the issue at hand.
template <class TL>
void dlist<TL>::insert_after(iterate iter, TL &item){
Dnode<TL> *inserta = new Dnode<TL>;
if(iter.current -> next() == NULL){
iter.current -> set_next(inserta);
inserta -> set_previous(iter.current);
//inserta -> set_next(NULL);
inserta -> set_data(item);
tail = inserta;
}
//else if(iter.current == head || iter.current == tail)
// rear_insert(item);
else {
inserta ->set_next(iter.current -> next());
inserta -> set_previous(iter.current);
inserta -> set_data(item);
iter.current -> next() -> set_previous(inserta);
iter.current -> set_next(inserta);
}
++used;
}
This function uses an iterator to get the current node position, and inserts a new node and data in the position after. When I compile my project I get a Seg Fault 11, and using the GDB run command, my code stops here
Program received signal SIGSEGV, Segmentation fault.
Dnode<Swatch>::next (this=0x0) at ./dnode.h:14
14 Dnode *next(){return nextptr;}
Stepping through it leads me here,
(gdb) s
main () at main2.cc:56
56 for(int i = 0;i < swatches.size()/2;i++)
(gdb) s
58 if(swatches.size()%2 == 1){
(gdb) s
62 swatches.insert_after(it,tmp);
(gdb) s
which leads me to believe the insert_after function is my problem. Also, here is my dnode class:
#ifndef DNODE_H
#define DNODE_H
template <class TL>
class Dnode{
public:
typedef TL value;
Dnode(const TL &item = TL(), Dnode *init_next = NULL, Dnode *init_prev = NULL)
{data1 = item; nextptr = init_next; previousptr = init_prev;}
TL &data(){return data1;}
Dnode *previous(){return previousptr;}
Dnode *next(){return nextptr;}
const Dnode *previous()const{return previousptr;}
const Dnode * next()const {return nextptr;}
void set_data(const TL &newData){data1 = newData;}
void set_previous(Dnode *newPrev){previousptr = newPrev;}
void set_next(Dnode *newNext){nextptr = newNext;}
private:
TL data1;
Dnode *nextptr, *previousptr;
};
#endif
So, if anybody can help me out i'd appreciate it. And if I need to include more, I will.
Thanks