fix: self.className.startsWith is not a function error fixed

Author: Alex-Sokolov

src/index.js

@@ -13,9 +13,9 @@ OverlayScrollbars.extension('scroll-chain', function (defaultOptions, framework,
         let foundViewport = false;
         let self = event.target;
         while (!foundViewport) {
-            if (!self)
+            if (!self) {
                 return false;
-            else if (self.className.startsWith('os-viewport')) {
+            } else if (self.className && self.className.startsWith('os-viewport')) {
                 foundViewport = true;
                 if (self !== instanceElements.viewport)
                     return false;