I believe this may be a duplicate of MC-177445 but the reason for the crash there is unknown so it's possible there are two separate issues here. Either way, it fails to mention the reason for the crash and no one has come forth with an interpretation fo the crash report as of yet so I've reported it here.

Running the command:

/give @p leather_leggings{AttributeModifiers:[{AttributeName:"generic.armor",Name:"generic.armor",Amount:1,Operation:0,UUIDLeast:172265,UUIDMost:296356,Slot:"legs"}]} 1

then attempting to look at the item will immediately crash your client. It seems that this is the case with every item with an attribute value, I would assume this is due to the most recent change to how attributes work. This can be found under the Attributes header of [this page|https://www.minecraft.net/en-us/article/minecraft-snapshot-20w14a].