chore: rewrite the exception message

The component may be attached, but the terminal-initialized event has not been fired yet.

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/xterm/XTermBase.java

@@ -194,7 +194,7 @@ private CompletableFuture<JsonValue> executeJs(boolean hasResult, String express
     } else if (deferredCommands == null) {
       return getElement().executeJs(expression, parameters).toCompletableFuture();
     } else {
-      throw new IllegalStateException("Component is not attached");
+      throw new IllegalStateException("Terminal is not initialized");
     }
   }