Fix bug: be compatible with removing root bone, which has no parent
Fix bug: be compatible with removing root bone, which has no parent

AlloyTeamDev/AlloyAnimation,kerryChen95/boneAnimationEditor,kerryChen95/boneAnimationEditor,AlloyTeam/AlloyAnimation,AlloyTeam/AlloyAnimation,AlloyTeamDev/AlloyAnimation,AlloyTeamDev/AlloyAnimation,AlloyTeam/AlloyAnimation,kerryChen95/boneAnimationEditor