Constructing an Binary Tree in java but just return a binarynode rather than binaryTree

Question

I am trying to implement binary tree in java Here is my code for my binary tree implementation.I am gettng an error please help: I get an Error:nullpointException ! In my opinion the function construstBinaryTree should return binary tree not just a root!!! Thank you for anwser my question.

''' public class Print_Array_Binary {

public static void main(String[] args) {
    int a[]={1,2, 3};
    TreeNode root = construstBinaryTree(a);
    System.out.println(root.left.val);// where I get  an Error:nullpointException !
    levelRead(root);
}
public static  TreeNode construstBinaryTree(int [] nums){
    TreeNode treeNodeArray[]=new TreeNode[nums.length];
    Arrays.fill(treeNodeArray,null);
    TreeNode root = null;
    for (int i = 0; i < nums.length;i++) {
      if(nums[i] != -1) {treeNodeArray[i] = new TreeNode(nums[i]);}
      if(i == 0) { root = new TreeNode(nums[i]);}
    }
    System.out.print("二叉结点数组：");
    for (TreeNode treeNode : treeNodeArray) {
        System.out.print(treeNode.val+" ");
    }
    System.out.println();
    //二叉树数组->二叉树
    for (int i = 0; 2 * i + 2 < nums.length; i++){
        if(treeNodeArray[i] != null) {
            treeNodeArray[i].left = treeNodeArray[2 * i + 1];
            treeNodeArray[i].right = treeNodeArray[2 * i + 2];
        }
    }
    System.out.println(treeNodeArray[0].left.val);
    System.out.println(treeNodeArray[0].right.val);

    return root;
}
public static void levelRead(TreeNode root)
{
    if(root == null) return;
    Queue<TreeNode> queue = new LinkedList<TreeNode>() ;
    queue.add(root);
    while(queue.size() != 0)
    {
        int len = queue.size();
        for(int i=0;i <len; i++)
        {
            TreeNode temp = queue.poll();
            System.out.print(temp.val+" ");
            if(temp.left != null)  queue.add(temp.left);
            if(temp.right != null) queue.add(temp.right);
        }
    }
}

}

'''