fix: png 16bit conversion crash, null guard added to handle if native returns null

lib/src/darwin/native.dart

@@ -212,8 +212,28 @@ final class ImageConverterDarwin implements ImageConverterPlatform {
         );
       }
 
-      final bitsPerComponent = CGImageGetBitsPerComponent(originalImage);
-      final bitmapInfo = CGImageGetBitmapInfo(originalImage);
+      // FIX: Always use 8 bits per component for the output bitmap context.
+      //
+      // The original code copied bitsPerComponent and bitmapInfo from the
+      // source CGImage. When the source is a 16-bit PNG, this produces a
+      // parameter combination that CGBitmapContextCreate does not support
+      // (e.g. RGB | 16 bits/component | kCGImageByteOrder16Little), causing
+      // it to return NULL. The subsequent CFRelease(NULL) in the finally
+      // block then triggers a fatal EXC_BREAKPOINT (brk #0x1).
+      //
+      // Since all output formats (JPEG, PNG, HEIC) produce 8-bit images,
+      // there is no need to create a 16-bit context. CGContextDrawImage()
+      // automatically downsamples 16-bit source pixels when drawing into
+      // an 8-bit context.
+      //
+      // kCGImageAlphaPremultipliedLast (value 1) is universally supported
+      // by CGBitmapContextCreate for all RGB colour spaces and correctly
+      // handles both opaque and transparent source images. For opaque
+      // output formats like JPEG, the alpha channel is discarded by the
+      // encoder.
+      const bitsPerComponent = 8;
+      // kCGImageAlphaPremultipliedLast = 1
+      const bitmapInfo = 1;
 
       context = CGBitmapContextCreate(
         nullptr,
@@ -251,7 +271,13 @@ final class ImageConverterDarwin implements ImageConverterPlatform {
       }
       return resizedImage;
     } finally {
-      if (context != null) CFRelease(context.cast());
+      // FIX: Check both Dart null AND FFI nullptr before releasing.
+      // The Dart variable `context` is non-null once assigned, but the
+      // native pointer may be nullptr if CGBitmapContextCreate failed.
+      // CFRelease(NULL) is a fatal error in CoreFoundation (brk #0x1).
+      if (context != null && context != nullptr) {
+        CFRelease(context.cast());
+      }
     }
   }
 }