WebXR: check for null render target (#15) (#17)

Author: idrisshah

src/renderers/WebGLRenderer.js

@@ -2306,7 +2306,7 @@ class WebGLRenderer {
 
 			renderTargetProperties.__autoAllocateDepthBuffer = depthTexture === undefined;
 
-			if ( ! renderTargetProperties.__autoAllocateDepthBuffer && ! _currentRenderTarget.isWebGLMultiviewRenderTarget ) {
+			if ( ! renderTargetProperties.__autoAllocateDepthBuffer && ( ! _currentRenderTarget || ! _currentRenderTarget.isWebGLMultiviewRenderTarget ) ) {
 
 				// The multisample_render_to_texture extension doesn't work properly if there
 				// are midframe flushes and an external depth buffer. Disable use of the extension.