Fix Popup crash in single window mode
Fix Popup crash in single window mode

focus_target->exclusive_child could be invalidated during the call to
focus_target->grab_focus(), now using the same logic with safe accesses
to focus_target.

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