fix(tui): guard Throbber DOM queries against unmounted state

src/cleveragents/tui/widgets/throbber.py

@@ -13,6 +13,7 @@ import random
 from enum import Enum
 
 from textual.app import ComposeResult
+from textual.css.query import NoMatches
 from textual.reactive import reactive
 from textual.timer import Timer
 from textual.widget import Widget
@@ -95,6 +96,8 @@ class Throbber(Widget):
 
     def _start_animation(self) -> None:
         self._stop_animation()
+        if not self.is_mounted:
+            return
         try:
             if self.style_mode == ThrobberStyle.RAINBOW:
                 self._timer = self.set_interval(_RAINBOW_FPS, self._tick_rainbow)
@@ -117,6 +120,10 @@ class Throbber(Widget):
 
     def _tick_rainbow(self) -> None:
         """Advance the rainbow gradient by one step."""
+        try:
+            content = self.query_one("#throbber-content", Static)
+        except NoMatches:
+            return
         width = self.size.width or 40
         gradient_len = len(RAINBOW_GRADIENT)
         segments: list[str] = []
@@ -124,7 +131,6 @@ class Throbber(Widget):
             color_idx = (i + self._offset) % gradient_len
             color = RAINBOW_GRADIENT[color_idx]
             segments.append(f"[{color}]{_GRADIENT_CHAR}[/]")
-        content = self.query_one("#throbber-content", Static)
         content.update("".join(segments))
         self._offset += 1
 
@@ -132,9 +138,12 @@ class Throbber(Widget):
         """Display the next loading quote."""
         if not self._shuffled_quotes:
             return
+        try:
+            content = self.query_one("#throbber-content", Static)
+        except NoMatches:
+            return
         self._current_quote = self._shuffled_quotes[
             self._quote_index % len(self._shuffled_quotes)
         ]
         self._quote_index += 1
-        content = self.query_one("#throbber-content", Static)
         content.update(f"[italic]{self._current_quote}[/italic]")