type 'null' is not a subtype of type 'string' of 'function result' flutter code example

Example: type 'double' is not a subtype of type 'String' of 'function result' dart

This will fix the error type 'int' is not a subtype of type 'double'
To fix type 'double' is not a subtype of type 'String', you need to find out 
the parameter which is returned as a double and you are trying to initialize 
it to a string variable.