Add send message validation for app-multiple-account.js

Just a simple example, for more validations example can be founded in app.js

app-multiple-account.js

@@ -164,12 +164,36 @@ io.on('connection', function(socket) {
 });
 
 // Send message
-app.post('/send-message', (req, res) => {
+app.post('/send-message', async (req, res) => {
   const sender = req.body.sender;
   const number = phoneNumberFormatter(req.body.number);
   const message = req.body.message;
 
-  const client = sessions.find(sess => sess.id == sender).client;
+  const client = sessions.find(sess => sess.id == sender)?.client;
+
+  // Make sure the sender is exists & ready
+  if (!client) {
+    return res.status(422).json({
+      status: false,
+      message: `The sender: ${sender} is not found!`
+    })
+  }
+
+  /**
+   * Check if the number is already registered
+   * Copied from app.js
+   * 
+   * Please check app.js for more validations example
+   * You can add the same here!
+   */
+  const isRegisteredNumber = await client.isRegisteredUser(number);
+
+  if (!isRegisteredNumber) {
+    return res.status(422).json({
+      status: false,
+      message: 'The number is not registered'
+    });
+  }
 
   client.sendMessage(number, message).then(response => {
     res.status(200).json({