Fix CanvasItem notification thread guard #12551

Job	Run time
📊 Static checks / Code style, file formatting, and docs	40s
🐧 Linux / Editor w/ Mono (target=editor)	4m 14s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	20m 39s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	7m 34s
🍎 macOS / Editor (target=editor, tests=yes)	5m 24s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	9m 20s
🍎 macOS / Template (target=template_release)	4m 2s
🤖 Android / Template (target=template_release)	3m 55s
🐧 Linux / Template w/ Mono (target=template_release)	1m 16s
🍏 iOS / Template (target=template_release)	3m 14s
🐧 Linux / Minimal template (target=template_release, everything disabled)	2m 41s
🌐 Web / Template (target=template_release)	1m 50s
🏁 Windows / Editor (target=editor, tests=yes)	13m 39s
🏁 Windows / Template (target=template_release)	6m 20s
🪲 Godot CPP / Build and test Godot CPP	11m 35s
	1h 36m 23s

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