[Bug] NullPointerException when renaming a method using setSimpleName on a VirtualFile

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Hi everyone,

I am unsure whether #3744 introduced a regression when calling setSimpleName on a model built from a VirtualFile. Is it valid to combine a VirtualFile and the SniperJavaPrettyPrinter?

Here is a minimal example that will throw a NullPointerException:

    @Test
    public void reproduction() {
        var resource = new VirtualFile("public class SomeClass { void SomeMethod() {}}");
        Launcher launcher = new Launcher();
        launcher.addInputResource(resource);
        launcher.getEnvironment().setPrettyPrinterCreator(
                () -> new spoon.support.sniper.SniperJavaPrettyPrinter(launcher.getEnvironment())
        );
        launcher.buildModel();
        var model = launcher.getModel();
        var executables = model.getElements(new TypeFilter<>(CtExecutable.class));
        var element = executables.get(1);
        element.setSimpleName("SomeOtherMethod");
    }

Exception location:
https://github.com/INRIA/spoon/pull/3744/files#diff-befbcc64ff160d86d9fa914487968edee1031311321a89dafbae1d0f69e4ee3bR254

spoon.support.sniper.internal.ElementSourceFragment.fromSameFile as the SourcePosition.getFile() call returns null.