  We challenge the view that there is a basic conflict between the fundamental
principles of Quantum Theory and General Relativity, and in particular the fact
that a superposition of massive bodies would lead to a violation of the
Equivalence Principle. It has been argued that this violation implies that such
a superposition must inevitably spontaneously collapse (like in the
Di\'osi-Penrose model). We identify the origin of such an assertion in the
impossibility of finding a local, classical reference frame in which Einstein's
Equivalence Principle would hold. In contrast, we argue that the formulation of
the Equivalence Principle can be generalised so that it holds for reference
frames that are associated to quantum systems in a superposition of spacetimes.
The core of this new formulation is the introduction of a quantum
diffeomorphism to such Quantum Reference Frames (QRFs). This procedure
reconciles the principle of linear superposition in Quantum Theory with the
principle of general covariance and the Equivalence Principle of General
Relativity. Hence, it is not necessary to invoke a gravity-induced spontaneous
state reduction when a massive body is prepared in a spatial superposition.
