Avoid iOS text selection crash by returning nil range (#161996)

[Previously approved](https://github.com/flutter/engine/pull/55909) in
/engine before monorepo shift.

Reverts an unnecessary assertion introduced by
[#16496](https://github.com/flutter/engine/pull/16496) that crashes upon
valid user text field interactions. Follow-on work can resolve the
underlying discrepancy between iOS and Flutter range finding (see
#160100). For crash reports, see issue
[#138464](https://github.com/flutter/flutter/issues/138464).

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Co-authored-by: Chris Bracken <chris@bracken.jp>

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.mm

@@ -1293,7 +1293,14 @@ static BOOL IsSelectionRectBoundaryCloserToPoint(CGPoint point,
   NSAssert([range isKindOfClass:[FlutterTextRange class]],
            @"Expected a FlutterTextRange for range (got %@).", [range class]);
   NSRange textRange = ((FlutterTextRange*)range).range;
-  NSAssert(textRange.location != NSNotFound, @"Expected a valid text range.");
+  if (textRange.location == NSNotFound) {
+    // Avoids [crashes](https://github.com/flutter/flutter/issues/138464) from an assertion
+    // against NSNotFound.
+    // TODO(hellohuanlin): This is a temp workaround, but we should look into why
+    // framework is providing NSNotFound to the engine.
+    // https://github.com/flutter/flutter/issues/160100
+    return nil;
+  }
   // Sanitize the range to prevent going out of bounds.
   NSUInteger location = MIN(textRange.location, self.text.length);
   NSUInteger length = MIN(self.text.length - location, textRange.length);

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputPluginTest.mm

@@ -530,6 +530,19 @@ FLUTTER_ASSERT_ARC
   XCTAssertEqual(substring.length, 0ul);
 }
 
+- (void)testTextInRangeAcceptsNSNotFoundLocationGracefully {
+  NSDictionary* config = self.mutableTemplateCopy;
+  [self setClientId:123 configuration:config];
+  NSArray<FlutterTextInputView*>* inputFields = self.installedInputViews;
+  FlutterTextInputView* inputView = inputFields[0];
+
+  [inputView insertText:@"text"];
+  UITextRange* range = [FlutterTextRange rangeWithNSRange:NSMakeRange(NSNotFound, 0)];
+
+  NSString* substring = [inputView textInRange:range];
+  XCTAssertNil(substring);
+}
+
 - (void)testStandardEditActions {
   NSDictionary* config = self.mutableTemplateCopy;
   [self setClientId:123 configuration:config];

engine/src/flutter/shell/platform/darwin/ios/framework/Source/TextInputSemanticsObject.mm

@@ -42,7 +42,14 @@ static const UIAccessibilityTraits kUIAccessibilityTraitUndocumentedEmptyLine =
   NSAssert([range isKindOfClass:[FlutterTextRange class]],
            @"Expected a FlutterTextRange for range (got %@).", [range class]);
   NSRange textRange = ((FlutterTextRange*)range).range;
-  NSAssert(textRange.location != NSNotFound, @"Expected a valid text range.");
+  if (textRange.location == NSNotFound) {
+    // Avoids [crashes](https://github.com/flutter/flutter/issues/138464) from an assertion
+    // against NSNotFound.
+    // TODO(hellohuanlin): This is a temp workaround, but we should look into why
+    // framework is providing NSNotFound to the engine.
+    // https://github.com/flutter/flutter/issues/160100
+    return nil;
+  }
   return [self.text substringWithRange:textRange];
 }