Added additional condition

Author: AZhurav1ev

enjoyhint.js

@@ -1437,8 +1437,10 @@
             else {
               distance = initial_distance;
               ver_button_position = initial_ver_position;
-              that.$next_btn.html(customBtnProps.nextButton ? customBtnProps.nextButton.text : 'Next');
-              that.$prev_btn.html(customBtnProps.prevButton ? customBtnProps.prevButton.text : 'Previous');
+			  that.$next_btn.html(customBtnProps.nextButton && customBtnProps.nextButton.text ? 
+					customBtnProps.nextButton.text : 'Next');
+			  that.$prev_btn.html(customBtnProps.prevButton && customBtnProps.prevButton.text ? 
+					customBtnProps.prevButton.text : 'Previous');
             }
 
             that.$prev_btn.css({