Updated react-native-macos version. ifdef'd out UIScrollViewDelegate methods for mac. Added an assert clause.

ios/RNCWebView.m

 @implementation RNCWKWebView
 - (void)scrollWheel:(NSEvent *)theEvent {
   RNCWebView *rncWebView = (RNCWebView *)[self superview];
+  RCTAssert([rncWebView isKindOfClass:[rncWebView class]], @"superview must be an RNCWebView");
   if (![rncWebView scrollEnabled]) {
     [[self nextResponder] scrollWheel:theEvent];
     return;
 #endif // !TARGET_OS_OSX
 
 #if !TARGET_OS_OSX
+// UIScrollViewDelegate method
 - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
-#else
-- (void)scrollViewWillBeginDragging:(RCTUIScrollView *)scrollView
-#endif // !TARGET_OS_OSX
 {
-#if !TARGET_OS_OSX
   scrollView.decelerationRate = _decelerationRate;
-#endif // !TARGET_OS_OSX
 }
+#endif // !TARGET_OS_OSX
 
 - (void)setScrollEnabled:(BOOL)scrollEnabled
 {
 }
 
 #if !TARGET_OS_OSX
+// UIScrollViewDelegate method
 - (void)scrollViewDidScroll:(UIScrollView *)scrollView
-#else
-- (void)scrollViewDidScroll:(RCTUIScrollView *)scrollView
-#endif // !TARGET_OS_OSX
 {
   // Don't allow scrolling the scrollView.
   if (!_scrollEnabled) {
     _onScroll(event);
   }
 }
+#endif // !TARGET_OS_OSX
 
 - (void)setDirectionalLockEnabled:(BOOL)directionalLockEnabled
 {

package.json

     "metro-react-native-babel-preset": "0.54.1",
     "react": "16.9.0",
     "react-native": "0.61.5",
-    "react-native-macos": "0.60.0-microsoft.44",
+    "react-native-macos": "0.60.0-microsoft.49",
     "semantic-release": "15.13.24",
     "typescript": "3.6.2"
   },

yarn.lock

   resolved "https://registry.yarnpkg.com/react-is/-/react-is-16.9.0.tgz#21ca9561399aad0ff1a7701c01683e8ca981edcb"
   integrity sha512-tJBzzzIgnnRfEm046qRcURvwQnZVXmuCbscxUO5RWrGTXpon2d4c8mI0D8WE6ydVIm29JiLB6+RslkIvym9Rjw==
 
-react-native-macos@0.60.0-microsoft.44:
-  version "0.60.0-microsoft.44"
-  resolved "https://registry.yarnpkg.com/react-native-macos/-/react-native-macos-0.60.0-microsoft.44.tgz#5d3e3edff7e282ea71526c1e167ca57d80ec9aff"
-  integrity sha512-QMb2cMyKAmAg+k2Io7af7SeOne9d1oUnl5Agbjy7YQe6/xmKl3qx0tM+mBvDJPZk204aqhmkvQJY8zpoVOyNsA==
+react-native-macos@0.60.0-microsoft.49:
+  version "0.60.0-microsoft.49"
+  resolved "https://registry.yarnpkg.com/react-native-macos/-/react-native-macos-0.60.0-microsoft.49.tgz#ac506343f7450f12f852769102e6262b531b181f"
+  integrity sha512-2631KIggGpw1RoLC/lWRZ/agFgAAMWuoQrRcmGsN+0neXHggEY5jUzz1uV6LeqKkrcudV+qaz3ajo85UjwqDIA==
   dependencies:
     "@babel/runtime" "^7.0.0"
     "@react-native-community/cli" "^2.6.0"