fix(QTooltip): guard MutationObserver against detached innerRef

[mathsgod]

What & Why

QTooltip.handleShow registers a MutationObserver inside a nextTick callback. If the component unmounts before that microtask runs (e.g. fast route change while hovering an anchor that triggers a tooltip show), innerRef.value is null and the call throws:

Uncaught (in promise) TypeError: Failed to execute 'observe' on 'MutationObserver': parameter 1 is not of type 'Node'.

The thrown error is unhandled and surfaces in the console. It is most visible in apps that pair QTooltip with quick navigation (table action buttons, tabs, dialogs).

Reproduction

1. Render a button with <q-tooltip>.
2. Hover the button so the tooltip enters its show flow.
3. Navigate to a new route before the next-tick callback runs (e.g. click a link in the same mouse-move tick).
4. Console shows the MutationObserver TypeError.

Fix

Add a null check and wrap observer.observe(...) in a try/catch. If the ref is detached when the tick fires, silently abort positioning instead of throwing. The component is on its way out anyway.

       registerTick(() => {
         observer = new MutationObserver(() => updatePosition())
-        observer.observe(innerRef.value, {
-          attributes: false,
-          childList: true,
-          characterData: true,
-          subtree: true
-        })
+        if (innerRef.value === null) {
+          observer = void 0
+          return
+        }
+        try {
+          observer.observe(innerRef.value, {
+            attributes: false,
+            childList: true,
+            characterData: true,
+            subtree: true
+          })
+        } catch (_e) {
+          observer = void 0
+        }
         updatePosition()
         configureScrollTarget()
       })

Notes

• The existing useTick composable already calls removeTick on onBeforeUnmount, so the pending tick is cancelled when possible. The null guard covers the remaining race where the tick fires after innerRef is detached but the VM is not yet considered destroyed (e.g. <keep-alive> deactivation, sibling unmount).
• No public API change.