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How to fix ERROR : invalid address (argument="address", value=undefined, code=INVALID_ARGUMENT, version=address/5.5.0)

I have completed the CryptoZombies‘s chapter6 tutorial, so I am challenged to work the front in my environment.

When I try to connect to the contract from the frontend I get an error message.

var cryptoZombies; var userAccount;  function startApp() {     var cryptoZombiesAddress = "0x8b87e80e2b76b4445a869b3dd0640e37c51def50";     cryptoZombies = new web3js.eth.Contract(cryptoZombiesABI, cryptoZombiesAddress);          async function getCurrentAccount(){       const web3 = new Web3(window.ethereum);       const userAccount = await web3.eth.getAccounts();       return userAccount;     }     getCurrentAccount(); }  function displayZombies(ids) {     $("#zombies").empty();     for (id of ids) {       // Look up zombie details from our contract. Returns a `zombie` object       getZombieDetails(id)       .then(function(zombie) {         // Using ES6's "template literals" to inject variables into the HTML.         // Append each one to our #zombies div         $("#zombies").append(`<div class="zombie">           <ul>             <li>Name: ${zombie.name}</li>             <li>DNA: ${zombie.dna}</li>             <li>Level: ${zombie.level}</li>             <li>Wins: ${zombie.winCount}</li>             <li>Losses: ${zombie.lossCount}</li>             <li>Ready Time: ${zombie.readyTime}</li>           </ul>         </div>`);       });     }  } function getZombiesByOwner(owner) {     return cryptoZombies.methods.getZombiesByOwner(owner).call(); }  window.addEventListener('load', function() {     // Checking if Web3 has been injected by the browser (Mist/MetaMask)     if (typeof web3 !== 'undefined') {       // Use Mist/MetaMask's provider       web3js = new Web3(window.ethereum);       alert("launch");     } else {       // Handle the case where the user doesn't have Metamask installed       // Probably show them a message prompting them to install Metamask       alert("please install metamask");     }     // Now you can start your app & access web3 freely:     startApp()     getZombiesByOwner(userAccount).then(displayZombies); })

I can get the current metamask account. However, When I try to run

getZombiesByOwner(userAccount).then(displayZombies);

it return an error.

invalid address (argument="address", value=undefined, code=INVALID_ARGUMENT, version=address/5.5.0) (argument="_owner", value=undefined, code=INVALID_ARGUMENT, version=abi/5.0.7)

Does a list of discrete log equations reveal information

Given public generator $g$ of some cyclic group, a secrets $xin Z_q$, and public pairs $(a_1,b_1),…,(a_n,b_n)$ that satisfies the equations $g^{x^{a_i}}=b_i$ for all $i=1,…,n$. Is it hard to compute to compute x (assuming the hardness of DLOG)? And a weaker version of the above problem, given a new group element $b_{i+1}$, is it hard to ::Listen

Given public generator $g$ of some cyclic group, a secrets $xin Z_q$, and public pairs $(a_1,b_1),…,(a_n,b_n)$ that satisfies the equations $g^{x^{a_i}}=b_i$ for all $i=1,…,n$.
Is it hard to compute to compute x (assuming the hardness of DLOG)?

And a weaker version of the above problem, given a new group element $b_{i+1}$, is it hard to compute an integer $a_{i+1}$ such $g^{x^{a_{i+1}}}=b_{i+1}$?