screen: Don't crash when first tab doesn't exist

while trying to attach a new client. Instead, check whether the first
tab does exist and if not, take the first tab index from the tabs
present in the session. If no tabs exist, panic with a better error
message.

zellij-server/src/screen.rs

@@ -702,21 +702,29 @@ impl Screen {
     }
 
     pub fn add_client(&mut self, client_id: ClientId) {
-        let mut tab_index = 0;
         let mut tab_history = vec![];
-        if let Some((_first_client, first_active_tab_index)) = self.active_tab_indices.iter().next()
-        {
-            tab_index = *first_active_tab_index;
-        }
         if let Some((_first_client, first_tab_history)) = self.tab_history.iter().next() {
             tab_history = first_tab_history.clone();
         }
+
+        let tab_index = if let Some((_first_client, first_active_tab_index)) =
+            self.active_tab_indices.iter().next()
+        {
+            *first_active_tab_index
+        } else if self.tabs.contains_key(&0) {
+            0
+        } else if let Some(tab_index) = self.tabs.keys().next() {
+            tab_index.to_owned()
+        } else {
+            panic!("Can't find a valid tab to attach client to!");
+        };
+
         self.active_tab_indices.insert(client_id, tab_index);
         self.connected_clients.borrow_mut().insert(client_id);
         self.tab_history.insert(client_id, tab_history);
         self.tabs
             .get_mut(&tab_index)
-            .unwrap()
+            .expect("Failed to attach client to tab with index {tab_index}")
             .add_client(client_id, None);
     }
     pub fn remove_client(&mut self, client_id: ClientId) {