Fixing the crash because of possibility for buttonStyles to be undefined, fixing the inconsistency in onAccessibilityTapIOS definition for Button and View. (#767)

src/common/Types.ts

@@ -641,7 +641,7 @@ export interface ViewPropsShared extends CommonProps, CommonAccessibilityProps {
     animateChildLeave?: boolean;
     animateChildMove?: boolean;
 
-    onAccessibilityTapIOS?: Function;
+    onAccessibilityTapIOS?: (e: SyntheticEvent) => void;
     onLayout?: (e: ViewOnLayoutEvent) => void;
     onMouseEnter?: (e: MouseEvent) => void;
     onMouseLeave?: (e: MouseEvent) => void;

src/web/Button.tsx

@@ -185,7 +185,7 @@ export class Button extends ButtonBase {
         }
 
         // Default to 'pointer' cursor for enabled buttons unless otherwise specified.
-        if (!buttonStyles.cursor) {
+        if (!buttonStyles || !buttonStyles.cursor) {
             buttonStyleMutations.cursor = this.props.disabled ? 'default' : 'pointer';
         }