[Tabs] Ignore selectedItem values not in items. (#7702)

When a client sets `selectedItem` to a value not already in `items`, the assignment should be ignored without error. This matches UITabBar's behavior.

Follow-up to #7656

components/Tabs/src/TabBarView/MDCTabBarView.m

@@ -82,12 +82,10 @@ - (void)setSelectedItem:(UITabBarItem *)selectedItem {
   }
 
   NSUInteger itemIndex = [self.items indexOfObject:selectedItem];
+  // Don't crash, just ignore if `selectedItem` isn't present in `_items`. This is the same behavior
+  // as UITabBar.
   if (selectedItem && (itemIndex == NSNotFound)) {
-    NSString *itemTitle = selectedItem.title;
-    NSString *exceptionMessage =
-        [NSString stringWithFormat:@"%@ is not a member of the tab bar's `items`.", itemTitle];
-    [[NSException exceptionWithName:NSInvalidArgumentException reason:exceptionMessage
-                           userInfo:nil] raise];
+    return;
   }
 
   _selectedItem = selectedItem;

components/Tabs/tests/unit/TabBarView/MDCTabBarViewTests.m

@@ -109,25 +109,18 @@ - (void)testSafelyHandlesEmptyItems {
   XCTAssertNil(self.tabBarView.selectedItem);
 }
 
-// Tab bar should throw error when select the item that doesn't belongs to items.
+// Tab bar should ignore setting a `selectedItem` to something not in the `items` array.
 - (void)testSafelyHandlesNonExistItem {
   // Given
   self.tabBarView.items = @[];
-  XCTAssertNil(self.tabBarView.selectedItem);
-
-  // When set selected item to nil.
   self.tabBarView.selectedItem = nil;
 
-  // Then should make no difference to the selection.
-  XCTAssertNil(self.tabBarView.selectedItem);
-
-  // Given items {A, B} which selected item A.
+  // When
   self.tabBarView.items = @[ self.itemA, self.itemB ];
-  self.tabBarView.selectedItem = self.itemA;
-  XCTAssertEqual(self.tabBarView.selectedItem, self.itemA);
+  self.tabBarView.selectedItem = self.itemC;
 
-  // When set selected item to C, then should raise exception.
-  XCTAssertThrows(self.tabBarView.selectedItem = self.itemC);
+  // Then
+  XCTAssertNil(self.tabBarView.selectedItem);
 }
 
 // Setting items to the same set of items should change nothing.