Try this... (note I'm using "exist" instead of "isexist")
clear % I'm starting with no 'h1'. This checks the "False" condition
if exist('h1')
delete(h1); % if 'h1' is a thing, delete it
end
h1=plot([5 8],[12 11]); % now there is an 'h1' either way
hold on; % I insertered this to match your application
pause(3); % wait a few seconds to verify it visually
% Let's do it again to check the "True" condition
if exist('h1')
delete(h1); % if 'h1' is a thing, delete it
end
h1 = plot([1 2],[2 4]); % still works
