Pressing enter at direct connect with an empty server address field will result in Minecraft trying to connect to a non-existing one (the empty ip). This isn't an issue in 1.12.2.
MC-157114 Pressing enter on direct connect when text box is empty still attempts to connect
MC-208154 UI text fields that aren't supposed to be left blank by the player can be bypassed
MC-122478 It is possible to create a server in the server list with a blank name and/or IP address
MC-156349 Cannot press Enter on Direct Connect
MC-207173 Entering only colons into multiplayer Direct Connection & pressing enter crashes the game