Sometimes you accidentally enter

clearclasses.m

clear classes

which then, inadvertently, also clears the contents of all displayed objects, which makes Matlab (due to the GUI's callbacks) go haywire and spit out tons of errors and warnings.

To get a true fresh start, you first need to close the GUI (without invoking any further callbacks). To do so, enter

clearfiguresandclasses.m

% get a handle to the xfigure class (default constructor)
xfig = xfigure;
 
% then delete all objects without further ado
xfig.DeleteAllFigures;
 
% and *then* get a fresh start
clear classes;