Fix a typo; set pugl backend as stub if it ends up being null

dgl/src/pugl.cpp

@@ -182,9 +182,11 @@ void puglSetMatchingBackendForCurrentBuild(PuglView* const view)
 #ifdef DGL_OPENGL
     puglSetBackend(view, puglGlBackend());
 #endif
-#ifdef DGL_Vulkan
+#ifdef DGL_VULKAN
     puglSetBackend(view, puglVulkanBackend());
 #endif
+    if (view->backend == nullptr)
+        puglSetBackend(view, puglStubBackend());
 }
 
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