Show me your code. So that I can give you a solution. For your reference: I am working on android project using ionic framework and angular. I encountered the error. Then I know that for subscriptionwithpriority method, the first parameter must be higher value. So that it will be your latest backbutton observer. Use the below code the override the default backbutton action.
this.platform.backbutton.subscribeWithPriority(9999, async () => {});