I recently solved this issue in my own code. My issue was caused by accidentally linking my javascript file, which calls initializeApp(), in the head and in the body of my html file. My fix was to delete the duplicate javascript tag in the head of my html file so only one existed in the body.
Scan through your code to make sure you only call initializeApp() once. This includes any js files that might be calling the method or duplicating the js file in your html file.
