Fix for crash on pre iOS 11 where the application delegate does not have a window variable (#772)

Author: korovyev

Branch-SDK/Branch-SDK/UIViewController+Branch.m

@@ -15,7 +15,9 @@ + (UIWindow*_Nullable) bnc_currentWindow {
     if (UIApplicationClass) {
         UIWindow *keyWindow = nil;
 
-        keyWindow = [UIApplicationClass sharedApplication].delegate.window;
+        if ([[UIApplicationClass sharedApplication].delegate respondsToSelector:@selector(window)]) {
+            keyWindow = [UIApplicationClass sharedApplication].delegate.window;
+        }
         if (keyWindow && !keyWindow.isHidden && keyWindow.rootViewController) return keyWindow;
 
         keyWindow = [UIApplicationClass sharedApplication].keyWindow;