Fix crash on startup

components/sceneutil/util.cpp

@@ -266,7 +266,9 @@ bool attachAlphaToCoverageFriendlyFramebufferToCamera(osg::Camera* camera, osg::
 {
 #if OSG_VERSION_LESS_THAN(3, 6, 6)
     // hack fix for https://github.com/openscenegraph/OpenSceneGraph/issues/1028
-    osg::GLExtensions::Get(0, false)->glRenderbufferStorageMultisampleCoverageNV = nullptr;
+    osg::ref_ptr<osg::GLExtensions> extensions = osg::GLExtensions::Get(0, false);
+    if (extensions)
+        extensions->glRenderbufferStorageMultisampleCoverageNV = nullptr;
 #endif
     unsigned int samples = 0;
     unsigned int colourSamples = 0;