Solutions :

Step 1 : Close the Browser Window & Clear the cookies and cache – Doing This Solves all the Problem of the bx Problem.

Step 2 : Another way ( only when you are changing Templates ) – Sometimes if there are a Lot of HTML widgets that are shown to be deleted, then Go Back to Layout > Page Templates and delete all the HTML Widgets. Now go to Edit Template to Update your Template. Even if the Problem persists Follow Step 1.