Fix crash if font fails loading when generating a preview
Fix crash if font fails loading when generating a preview

Historical crash log:

ERROR: No loader found for resource: res://addons/gdquest.mannequin/assets/theme/fonts/montserrat/Montserrat-Medium.ttf.
   at: (core\io\resource_loader.cpp:213)
ERROR: res://addons/gdquest.mannequin/assets/theme/fonts/default_font.tres:8 - Parse Error: [ext_resource] referenced non-loaded resource at: res://addons/gdquest.mannequin/assets/theme/fonts/montserrat/Montserrat-Medium.ttf
   at: ResourceLoaderText::_parse_ext_resource (scene\resources\resource_format_text.cpp:170)
ERROR: res://addons/gdquest.mannequin/assets/theme/fonts/default_font.tres:8 - Parse Error: [ext_resource] referenced non-loaded resource at: res://addons/gdquest.mannequin/assets/theme/fonts/montserrat/Montserrat-Medium.ttf
   at: ResourceLoaderText::load (scene\resources\resource_format_text.cpp:649)
ERROR: Failed loading resource: res://addons/gdquest.mannequin/assets/theme/fonts/default_font.tres. Make sure resources have been imported by opening the project in the editor at least once.
   at: (core\io\resource_loader.cpp:206)

vkbsb/godot,Valentactive/godot,vnen/godot,Zylann/godot,Faless/godot,godotengine/godot,sanikoyes/godot,Faless/godot,josempans/godot,Shockblast/godot,BastiaanOlij/godot,sanikoyes/godot,godotengine/godot,akien-mga/godot,sanikoyes/godot,Faless/godot,sanikoyes/godot,josempans/godot,godotengine/godot,vnen/godot,josempans/godot,firefly2442/godot,vnen/godot,Shockblast/godot,vkbsb/godot,pkowal1982/godot,godotengine/godot,guilhermefelipecgs/godot,Faless/godot,vkbsb/godot,pkowal1982/godot,Valentactive/godot,BastiaanOlij/godot,Valentactive/godot,sanikoyes/godot,Faless/godot,ZuBsPaCe/godot,guilhermefelipecgs/godot,akien-mga/godot,pkowal1982/godot,akien-mga/godot,pkowal1982/godot,Shockblast/godot,Valentactive/godot,guilhermefelipecgs/godot,ZuBsPaCe/godot,Faless/godot,BastiaanOlij/godot,akien-mga/godot,pkowal1982/godot,josempans/godot,vnen/godot,Faless/godot,firefly2442/godot,ZuBsPaCe/godot,BastiaanOlij/godot,Shockblast/godot,vkbsb/godot,godotengine/godot,firefly2442/godot,BastiaanOlij/godot,Shockblast/godot,vkbsb/godot,godotengine/godot,guilhermefelipecgs/godot,akien-mga/godot,Valentactive/godot,pkowal1982/godot,sanikoyes/godot,vnen/godot,godotengine/godot,josempans/godot,Zylann/godot,pkowal1982/godot,BastiaanOlij/godot,vkbsb/godot,Shockblast/godot,akien-mga/godot,akien-mga/godot,firefly2442/godot,BastiaanOlij/godot,Zylann/godot,vkbsb/godot,josempans/godot,Faless/godot,josempans/godot,guilhermefelipecgs/godot,Shockblast/godot,Zylann/godot,josempans/godot,firefly2442/godot,sanikoyes/godot,firefly2442/godot,guilhermefelipecgs/godot,Valentactive/godot,Valentactive/godot,BastiaanOlij/godot,ZuBsPaCe/godot,Zylann/godot,Zylann/godot,sanikoyes/godot,ZuBsPaCe/godot,vnen/godot,ZuBsPaCe/godot,vnen/godot,vnen/godot,ZuBsPaCe/godot,godotengine/godot,Zylann/godot,akien-mga/godot,pkowal1982/godot,guilhermefelipecgs/godot,firefly2442/godot,ZuBsPaCe/godot,guilhermefelipecgs/godot,Zylann/godot,firefly2442/godot,Shockblast/godot,vkbsb/godot,Valentactive/godot