Use of default constructor in struct datatype C++ [duplicate]

Why do I need to define a new Trienode in the following code as new TrieNode()? When I do it as new TrieNode, it gives a segmentation fault. Is there a default constructor for a struct datatype that's important to call here, if so what does it do?

struct TrieNode 
{
    struct TrieNode *children[ALPHABET_SIZE];
    bool isLeaf;
};
void insert(struct TrieNode *root, string key)
{
    for(int i=0; i<key.size(); i++)
    {
        if(root->children[key[i]-'a'] ==NULL)   root->children[key[i]-'a'] = new TrieNode();
        root = root->children[key[i]-'a'];
    }
    root->isLeaf = true;
        
}