Prevent calling `get_material` for invalid gizmo #569

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 0s
🍏 iOS / Template (target=template_release)	8m 57s
🍎 macOS / Editor (target=editor, tests=yes)	29m 21s
🤖 Android / Editor (target=editor)	24m 41s
🍎 macOS / Template (target=template_release)	20m 40s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	16m 26s
🐧 Linux / Editor w/ Mono (target=editor)	40m 33s
🐧 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)	44m 25s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	16m 33s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	36m 28s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	25m 8s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	32m 6s
🏁 Windows / Editor (target=editor, tests=yes)	50m 29s
🌐 Web / Template w/o threads (target=template_release, threads=no)	26m 31s
🏁 Windows / Editor w/ clang-cl (target=editor, tests=yes, use_llvm=yes)	44m 53s
🐧 Linux / Template w/ Mono (target=template_release)	27m 24s
🏁 Windows / Template (target=template_release)	37m 17s
🐧 Linux / Minimal template (target=template_release, everything disabled)	15m 34s
🪲 Godot CPP / Build and test Godot CPP	8m 13s
	8h 26m 39s

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