fix(iOS): Add null check for fontName in RCTGetFontWeight to prevent crash

[25harsh]

Summary:

Adds defensive null check for fontName in RCTGetFontWeight before passing it to CFStringFind(), preventing EXC_BAD_ACCESS crash.

Problem

RCTGetFontWeight passes font.fontName directly to CFStringFind() without checking if the result is nil. Unlike Objective-C methods, Core Foundation functions like CFStringFind do not gracefully handle NULL pointers and will
crash with EXC_BAD_ACCESS.

While standard React Native apps typically register fonts at compile time. There are legitimate scenarios where fonts may not be immediately available. eg.
Brownfield applications - runtime font registration using CTFontManagerRegisterGraphicsFont, this causes RN to crash

Changelog:

[IOS][FIXED] - Added null check for fontName in RCTGetFontWeight to prevent crash

Test Plan:

When calling RCTGetFontWeight with nil UIFont (either empty string or unregistered font), no crash observed.

[25harsh]

@cipolleschi can you please check this

[isaacrowntree]

We're hitting this exact crash in production on React Native 0.83.2 with Fabric enabled (iOS).

Crash stack from Crashlytics:

Thread 11 Crashed:
0  CoreFoundation    CF_IS_OBJC + 12
1  CoreFoundation    CFStringGetLength + 232
2  CoreFoundation    CFStringFind + 60
3  React             RCTGetFontWeight + 212
4  React             RCTFontWithFontProperties + 3024
5  React             RCTNSTextAttributesFromTextAttributes + 316
6  React             RCTNSAttributedStringFromAttributedString + 364
7  React             -[RCTTextLayoutManager measureAttributedString:...] + 76

Happens during concurrent Yoga layout measurement on the Hermes JS thread when a UIFont fails to resolve. We've applied this same nil guard locally via patch-package as a workaround. Would love to see this merged — it's a one-liner that prevents a fatal SIGSEGV.

[isaacrowntree]

We're hitting this same crash in production on RN 0.83.2 with Fabric enabled (posted a comment here previously with our Crashlytics stack).

One thing worth noting: your PR guards fontName but not the font parameter itself. We've applied both guards locally via patch-package and would suggest adding a nil check for font at the top of the function as well:

RCTFontWeight RCTGetFontWeight(UIFont *font)
{
  if (!font) {
    return UIFontWeightRegular;
  }
  // ... existing code ...
  NSString *fontName = font.fontName;
  if (fontName) {
    // ... suffix matching loop ...
  }

  auto traits = (__bridge_transfer NSDictionary *)CTFontCopyTraits((CTFontRef)font);
  return (RCTFontWeight)[traits[UIFontWeightTrait] doubleValue];
}

If font is nil, font.fontName returns nil (ObjC message to nil), which then hits the same CFStringFind(NULL, ...) SIGSEGV. The !font guard is defense-in-depth — while the fontName guard catches the case we're seeing (non-nil UIFont with a nil fontName from a partially-resolved descriptor during concurrent Fabric layout), guarding font too makes the function robust against both scenarios.

[25harsh]

hey @isaacrowntree thanks for the suggestion. I've gone ahead and added the !font guard at the top as well, so both scenarios are now covered.

@cipolleschi would you be able to take a look when you get a chance? Would be great to get this landed given the production crashes folks are seeing.