fix(lua.endpoint): add nil checks for race condition in next_round endpoint

The condition in next_round.lua was accessing G.blind_select_opts
immediately when G.STATE became BLIND_SELECT, but the UI elements
are created asynchronously via Events in update_blind_select().

This caused crashes when loading saves from SHOP state because
G.blind_select_opts wasn't populated yet when the state changed.

Now properly waits for each component to be initialized:
- G.STATE to be BLIND_SELECT
- G.blind_select_opts table to exist
- The specific blind pane (small/big/boss) to exist
- The select button UI element to exist

Author: S1M0N38

src/lua/endpoints/next_round.lua

@@ -32,14 +32,28 @@ return {
       trigger = "condition",
       blocking = false,
       func = function()
-        local blind_pane = G.blind_select_opts[string.lower(G.GAME.blind_on_deck)]
+        -- Wait for state transition and UI to be fully initialized
+        if G.STATE ~= G.STATES.BLIND_SELECT then
+          return false
+        end
+        if not G.blind_select_opts then
+          return false
+        end
+
+        local blind_key = string.lower(G.GAME.blind_on_deck)
+        local blind_pane = G.blind_select_opts[blind_key]
+        if not blind_pane then
+          return false
+        end
+
         local select_button = blind_pane:get_UIE_by_ID("select_blind_button")
-        local done = G.STATE == G.STATES.BLIND_SELECT and select_button ~= nil
-        if done then
-          sendDebugMessage("Return next_round() - reached BLIND_SELECT state", "BB.ENDPOINTS")
-          send_response(BB_GAMESTATE.get_gamestate())
+        if not select_button then
+          return false
         end
-        return done
+
+        sendDebugMessage("Return next_round() - reached BLIND_SELECT state", "BB.ENDPOINTS")
+        send_response(BB_GAMESTATE.get_gamestate())
+        return true
       end,
     }))
   end,