Fix null shadow color crash

Author: gabrieldonadel

packages/react-native/React/Views/RCTView.m

@@ -764,7 +764,10 @@ - (void)setShadowRadius:(CGFloat)shadowRadius
 -(void)didUpdateShadow
 {
     NSShadow *shadow = [NSShadow new];
-    NSColor *baseShadowColor = [NSColor colorWithCGColor:_shadowColor];
+    NSColor *baseShadowColor = nil;
+    if (_shadowColor != NULL) {
+      baseShadowColor = [NSColor colorWithCGColor:_shadowColor];
+    }
     shadow.shadowColor = [baseShadowColor colorWithAlphaComponent:[self shadowOpacity]];
     shadow.shadowOffset = [self shadowOffset];
     shadow.shadowBlurRadius = [self shadowRadius];