Fix is_inside_tree error with CPUParticles3D #25

Job	Run time
📊 Static checks / Code style, file formatting, and docs	30s
🍎 macOS / Editor (target=editor, tests=yes)	28m 30s
🤖 Android / Editor (target=editor)	5m 26s
🍎 macOS / Template (target=template_release, tests=yes)	24m 0s
🍏 iOS / Template (target=template_release)	1m 35s
🐧 Linux / Editor w/ Mono (target=editor)	4m 25s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	3m 35s
🐧 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)	11m 37s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 43s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	7m 9s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 9s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	6m 36s
🏁 Windows / Editor (target=editor, tests=yes)	8m 22s
🌐 Web / Template w/o threads (target=template_release, threads=no)	6m 50s
🐧 Linux / Template w/ Mono (target=template_release, tests=yes)	2m 8s
🏁 Windows / Editor w/ clang-cl (target=editor, tests=yes, use_llvm=yes)	4m 19s
🐧 Linux / Minimal template (target=template_release, tests=yes, everything disabled)	2m 10s
🏁 Windows / Template (target=template_release, tests=yes)	7m 59s
🪲 Godot CPP / Build and test Godot CPP	9m 3s
	2h 24m 6s

Provide feedback