How can I output a sequence as post order?

Question

Question description: I want to output a binary tree as post order, but the program occurred in a runtime error. I don't know where the problem occurred. And I don't know how to solve it everytime I occurred in runtime error, unlike compile error. By the way, what books or articles should I read about solving runtime error?

MainFunc.c

#include "AllFun.h"
int main(int argc, char* argv[])
{
    BiTree T;
    T = create();
    /* Traverse nonrecursively binary tree as post order */
    PostOrder(T);
    return 0;
}

AllFun.h

#include <stddef.h>   // NULL
#include <stdlib.h>   //malloc()
#include <stdio.h>
#define MaxSize 10
typedef int ElemType;
typedef struct BiTNode {
    ElemType data;
    struct BiTNode* lchild, * rchild;
} BiTNode, * BiTree;
typedef struct Stack {
    ElemType data[MaxSize];
    int top;
} SqStack;
BiTree create();
void PostOrder(BiTree T);

OtheFunc.c

#include "AllFun.h"
BiTree newNode(int value)
{
    BiTree T = (BiTNode*)malloc(sizeof(BiTNode));
    if (T) {
        T->data = value;
        T->lchild = T->rchild = NULL;
    }
    return T;
}
void insert(BiTree* T, int x)
{
    if (*T == NULL) {
        *T = newNode(x);
        return;
    }
    if (x < (*T)->data) {
        insert(&(*T)->lchild, x);
    }
    else {
        insert(&(*T)->rchild, x);
    }
}
BiTree create()
{
    ElemType nums[MaxSize];
    for (int i = 0; i < MaxSize; ++i) {
        nums[i] = ' ';
    }
    BiTree T = NULL;
    printf("Enter value of node to create a binary search tree: ");
    for (int i = 0; i < MaxSize; ++i) {
        scanf_s("%d", &nums[i]);
        insert(&T, nums[i]);
    }
    return T;
}
void init(SqStack *s)
{
    s->top = -1;
}
int isEmpty(SqStack *s)
{
    int ret = 0;
    if (s->top == -1) {
        ret = 1;
    }
    return ret;
}
void push(SqStack *s, BiTree p)
{
    if (s->top == MaxSize - 1) {
        printf("stack is full\n");
    }
    else {
        s->data[++s->top] = p->data;
    }
}
void getTop(SqStack *s, BiTree p)
{
    if (s->top == -1) {
        printf("stack is empty\n");
        return;
    }
    p->data = s->data[s->top];
}
void pop(SqStack *s, BiTree p)
{
    if (s->top == -1) {
        printf("stack is empty\n");
    }
    p->data = s->data[s->top--];
}
void PostOrder(BiTree T)
{
    BiTree p = T, r = NULL;
    SqStack s;
    init(&s);
    while (p || !isEmpty(&s)) {
        if (p) {
            push(&s, p);
            p = p->lchild;
        }
        else {
            getTop(&s, p);
            if (p) {
                if (p->rchild && p->rchild != r) {
                    p = p->rchild;
                    push(&s, p);
                    p = p->lchild;
                }
            }
            else {
                pop(&s, p);
                if (p) {
                    printf("%d ", p->data);
                }
                r = p;
                p = NULL;
            }
        }
    }// while
}