AbiWord uses programming libraries named Clutter, GTK+, and Clutter-GTK. The latter library glues the former twain together. One of its the necessary parts is a call to a function named
gtk_clutter_init(). This returns an error code if initialization fails, as it indeed does if there's no display.
Programs that ignore the result from calling this function, and carry on regardless of potential error, emit compiler warnings when compiled. Programmers have addressed this in different ways. Colin Watson made GNOME Shell emit a fairly bland fatal error message. Hubert Figuière made AbiWord emit the message that you see before you. Neither message is really helpful to the end user, who really needs to be told that there is a problem of some kind initializing the GUI, and probably won't know what "Clutter" is referring to.
Interestingly, the result code is zero according to your error message. According to the doco an error result is supposed to be a non-zero negative integer. In fact, there's an undocumented "unknown" error condition, which is neither success (which has the value 1) nor a negative number failure code; which arises as a result of the
gtk_init_check() library function being unable to initialize the GUI.
** (abiword:1262): WARNING **: Could not open X display,
Failed to connect to Mir: Failed to connect to server socket: No such file or directory,
Unable to init server: Could not connect: Connection refused. That describes the actual problem. The message you're wondering about comes after that, apparently because the code that handles Abiword's exit after recognizing that there is no usable display would need some developer attention. I guess a perfectly clean handling of a "no display" error is low priority for the Abiword devs.