get_transpiler() crashes if called before get_backend()

[BrunoLiegiBastonLiegi]

This fails with a AttributeError: 'NoneType' object has no attribute 'qubits' for me

from qibo import get_transpiler, get_backend

get_transpiler()
get_backend()

whereas the opposite works

get_backend()
get_transpiler()

presumably because get_backend sets some kind of global variable that is used by get_transpiler.

[alecandido]

It does (though everything is local to the _Global class, which on its turn is global - making everything global as a consequence).

The solution is just to replace

qibo/src/qibo/backends/__init__.py

Lines 138 to 139 in 4588671

	qubits = cls._backend.qubits
	natives = cls._backend.natives

with a call to get_backend, i.e.

    qubits = cls.backend().qubits
    natives = cls.backend().natives

In this way, the second invocation will get the actual value set, and the first one will set it, only if there is none yet.