Completed BMI APP with up to date flutter. converted to V2 embedding and Null Safety implementation

[mhaseebali]

hey there :)

this is updated code for the BMI APP, while the original has issues such as it needs to be converted to V2 embedding and null safety has to be implemented due to newer concepts, here is the whole code so you won't run into any sort of problems!

Issues resolved

requires your app to be migrated to the Android embedding v2

Null safety implementation

void call by functions or variables where values are to be inserted on runtime can cause issues if no null safety is used in flutter, it has been resolved in this updated version of the code