Prevent calling requestsDismissalOfViewController

This fix prevents calling requestsDismissalOfViewController when there
is nothing to dismiss. The same way
`requestsPresentationOfViewController` is not being called.

This is also crashing on runtime on swift as this is passing `nil` to a
function signature that is mark as `non_null`.

BraintreePayPal/BTPayPalDriver.m

@@ -317,7 +317,10 @@ - (void)requestExpressCheckout:(BTPayPalRequest *)request
 - (void)setAppSwitchReturnBlock:(void (^)(BTPayPalAccountNonce *tokenizedAccount, NSError *error))completionBlock
                  forPaymentType:(BTPayPalPaymentType)paymentType {
     appSwitchReturnBlock = ^(NSURL *url) {
-        [self informDelegatePresentingViewControllerNeedsDismissal];
+        if (self.safariViewController) {
+            [self informDelegatePresentingViewControllerNeedsDismissal];
+        }
+
         [self informDelegateWillProcessAppSwitchReturn];
 
         // Before parsing the return URL, check whether the user cancelled by breaking