In the context of programming, objects have two states where null = uninitialized (the other state is initialized) - when an object is null it means that it basically does not exist in any way. When you use JSON, if the object you are trying to deserialize is not formatted correctly most serializers will set the object to null instead of throwing an error.
In general, with where you are at right now I would search the following on google:
[Your language> - null handling
[Your Language> - Web Request
[Your Language> - Deserialize JSON
[Your Language> - Echo Object Values
From your error, your $team = blahblah is probably line 139, I suspect $data is null.