Added possible cause to exception message. (#1307)

* Added possible cause to exception message.

* Updated message.

* Updated exception message.

Author: DevJasperNL

src/HassModel/NetDaemon.HassModel/Internal/EntityStateCache.cs

@@ -52,8 +52,12 @@ private void HandleEvent(HassEvent hassEvent)
     public EntityState? GetState(string entityId)
     {
         return !_initialized
-            ? throw new InvalidOperationException("StateCache has not been initialized yet")
-            : (_latestStates.GetValueOrDefault(entityId)?.Value);
+            ? throw new InvalidOperationException(
+                "StateCache has not been initialized. This may occur if NetDaemon has not yet completed its initial connection to Home Assistant. " +
+                "This should not happen with a standard NetDaemon deployment. If you're using a custom deployment, ensure that initialization is complete " +
+                "before accessing entity state by awaiting `WaitForInitializationAsync` on `INetDaemonRuntime`."
+            )
+            : _latestStates.GetValueOrDefault(entityId)?.Value;
     }
 
     public void Dispose()