Add a guard around first attempt at setPoseTarget

Author: stlewis

src/core/scene/a-scene.js

@@ -547,6 +547,7 @@ module.exports.AScene = registerElement('a-scene', {
         renderer = this.renderer = new THREE.WebGLRenderer(rendererConfig);
         renderer.setPixelRatio(window.devicePixelRatio);
         renderer.sortObjects = false;
+        if(this.camera) renderer.vr.setPoseTarget(self.camera.el.object3D);
         this.addEventListener('camera-set-active', function () {
           renderer.vr.setPoseTarget(self.camera.el.object3D);
         });