Fix TooltipState null check error (#106330)

packages/flutter/lib/src/material/tooltip.dart

@@ -529,7 +529,7 @@ class TooltipState extends State<Tooltip> with SingleTickerProviderStateMixin {
   /// Returns `false` when the tooltip shouldn't be shown or when the tooltip
   /// was already visible.
   bool ensureTooltipVisible() {
-    if (!_visible) {
+    if (!_visible || !mounted) {
       return false;
     }
     _showTimer?.cancel();

packages/flutter/test/material/tooltip_test.dart

@@ -1053,6 +1053,37 @@ void main() {
     gesture = null;
   });
 
+  testWidgets('Calling ensureTooltipVisible on an unmounted TooltipState returns false', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/95851
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Center(
+          child: Tooltip(
+            message: tooltipText,
+            child: SizedBox(
+              width: 100.0,
+              height: 100.0,
+            ),
+          ),
+        ),
+      ),
+    );
+
+    final TooltipState tooltipState = tester.state(find.byType(Tooltip));
+    expect(tooltipState.ensureTooltipVisible(), true);
+
+    // Remove the tooltip.
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Center(
+          child: SizedBox.shrink(),
+        ),
+      ),
+    );
+
+    expect(tooltipState.ensureTooltipVisible(), false);
+  });
+
   testWidgets('Tooltip shows/hides when hovered', (WidgetTester tester) async {
     const Duration waitDuration = Duration.zero;
     TestGesture? gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);