Fix `FileSystemDock` thumbnails sometimes not displaying #29279

Job	Run time
📊 Static checks / Code style, file formatting, and docs	31s
🍏 iOS / Template (target=template_release)	2m 20s
🤖 Android / Template (target=template_release)	23m 21s
🐧 Linux / Editor w/ Mono (target=editor)	32m 42s
🍎 macOS / Editor (target=editor, tests=yes)	25m 36s
🐧 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)	42m 16s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	28m 43s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	17m 34s
🍎 macOS / Template (target=template_release)	14m 55s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	27m 19s
🌐 Web / Template w/o threads (target=template_release, threads=no)	17m 25s
🐧 Linux / Template w/ Mono (target=template_release)	18m 41s
🐧 Linux / Minimal template (target=template_release, everything disabled)	12m 10s
🏁 Windows / Editor (target=editor, tests=yes)	38m 25s
🏁 Windows / Template (target=template_release)	27m 25s
🪲 Godot CPP / Build and test Godot CPP	7m 7s
	5h 36m 30s

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