using just a single column for a comboBox editor, I can reproduce your problem
only if the last row is selected/deleted, then another selected after deletion.
adding this line seems to fix the problem
myjTable.putClientProperty("terminateEditOnFocusLost", Boolean.TRUE);
as an aside,
you should be using an actionListener for your button instead of a mouseListener,
it's no big deal, just consistency, but if you prefer mouseListener, note that mouseClicked
is definitely not the right choice, as it doesn't always fire. mousePressed is a far better choice.
to
test mouseClicked, click and hold, move the mousePointer just a bit, now release. the event won't fire.