The game instantly crashes on (in my opinion) correct Entity NBT data.
For example:

/tellraw @a {"text":"Hover me and you crash!","hoverEvent":{"action":"show_entity","value":"foo"}}

will, as expected, say "Invalid Entity"

however if a NBT-format is used (for example:

/tellraw @a {"text":"Hover me and you crash!","hoverEvent":{"action":"show_entity","value":"{fire:-1s}"}}

), Minecraft will crash wether a Creeper exists or not. This happens with every NBT-Format within the quotation-marks. A Null Pointer exception will be thrown.

This is not the only case though... All commands with some kind of NBT-Format-Text in quotation-marks will cause Minecraft to crash

(Additional Crash information below)