fix missing cleanup and null checks for various singletons

servers/rendering/renderer_compositor.cpp

@@ -47,6 +47,7 @@ bool RendererCompositor::is_xr_enabled() const {
 }
 
 RendererCompositor::RendererCompositor() {
+	ERR_FAIL_COND_MSG(singleton != nullptr, "A RendererCompositor singleton already exists.");
 	singleton = this;
 
 #ifndef _3D_DISABLED
@@ -57,3 +58,7 @@ RendererCompositor::RendererCompositor() {
 	}
 #endif // _3D_DISABLED
 }
+
+RendererCompositor::~RendererCompositor() {
+	singleton = nullptr;
+}