Fix Nullabilty warnings: Implicit conversion from nullable pointer 'X' to non-nullable pointer type 'Y'

[sadefigu]

Building the project with the latest version of XCode generates 10 nullability warnings like the below:

Implicit conversion from nullable pointer 'id _Nullable' to non-nullable pointer type 'NSString * _Nonnull'

[sadefigu]

PR #504 should fix this issue.