fix missing cleanup and null checks for various singletons

editor/editor_node.cpp

@@ -3537,7 +3537,7 @@ void EditorNode::add_extension_editor_plugin(const StringName &p_class_name) {
 
 void EditorNode::remove_extension_editor_plugin(const StringName &p_class_name) {
 	// If we're exiting, the editor plugins will get cleaned up anyway, so don't do anything.
-	if (singleton->exiting) {
+	if (!singleton || singleton->exiting) {
 		return;
 	}
 

editor/plugins/node_3d_editor_plugin.cpp

@@ -8431,6 +8431,7 @@ Node3DEditor::Node3DEditor() {
 	VBoxContainer *vbc = this;
 
 	custom_camera = nullptr;
+	ERR_FAIL_COND_MSG(singleton != nullptr, "A Node3DEditor singleton already exists.");
 	singleton = this;
 	editor_selection = EditorNode::get_singleton()->get_editor_selection();
 	editor_selection->add_editor_plugin(this);
@@ -9060,6 +9061,7 @@ void fragment() {
 	clear(); // Make sure values are initialized. Will call _snap_update() for us.
 }
 Node3DEditor::~Node3DEditor() {
+	singleton = nullptr;
 	memdelete(preview_node);
 	if (preview_sun_dangling && preview_sun) {
 		memdelete(preview_sun);

servers/rendering/renderer_canvas_render.h

@@ -545,8 +545,13 @@ public:
 
 	virtual void set_debug_redraw(bool p_enabled, double p_time, const Color &p_color) = 0;
 
-	RendererCanvasRender() { singleton = this; }
-	virtual ~RendererCanvasRender() {}
+	RendererCanvasRender() {
+		ERR_FAIL_COND_MSG(singleton != nullptr, "A RendererCanvasRender singleton already exists.");
+		singleton = this;
+	}
+	virtual ~RendererCanvasRender() {
+		singleton = nullptr;
+	}
 };
 
 #endif // RENDERER_CANVAS_RENDER_H

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;
+}

servers/rendering/renderer_compositor.h

@@ -110,7 +110,7 @@ public:
 
 	static RendererCompositor *get_singleton() { return singleton; }
 	RendererCompositor();
-	virtual ~RendererCompositor() {}
+	virtual ~RendererCompositor();
 };
 
 #endif // RENDERER_COMPOSITOR_H

servers/rendering/renderer_rd/renderer_compositor_rd.cpp

@@ -299,6 +299,7 @@ RendererCompositorRD::RendererCompositorRD() {
 		}
 	}
 
+	ERR_FAIL_COND_MSG(singleton != nullptr, "A RendererCompositorRD singleton already exists.");
 	singleton = this;
 
 	utilities = memnew(RendererRD::Utilities);
@@ -330,6 +331,7 @@ RendererCompositorRD::RendererCompositorRD() {
 }
 
 RendererCompositorRD::~RendererCompositorRD() {
+	singleton = nullptr;
 	memdelete(uniform_set_cache);
 	memdelete(framebuffer_cache);
 	ShaderRD::set_shader_cache_dir(String());