Created attachment 4349 
possible patch

Hi,

Xfce4-sensors-plugin crashes when I try to remove it from the panel here, with a trace top similar to:
 __GI___libc_free (mem=0x7fb5ff738704) at malloc.c:2982
 sensors_free (plugin=<optimized out>, sensors=0x7fb600a17390) at sensors-plugin.c:1091

Looking at the code, sensors->font_size is sometimes used to hold strdup'd strings, and sometimes static strings. So it won't go well when it tries to free it in sensors_free().

lib/sensors-interface-common.c:    sensors->font_size = "medium";
lib/configuration.c:            sensors->font_size = g_strdup(value);
panel-plugin/sensors-plugin.c:    g_free (sensors->font_size);
panel-plugin/sensors-plugin.c:        case 0: sd->sensors->font_size = "x-small"; break;
panel-plugin/sensors-plugin.c:        case 1: sd->sensors->font_size = "small"; break;
panel-plugin/sensors-plugin.c:        case 3: sd->sensors->font_size = "large"; break;
panel-plugin/sensors-plugin.c:        case 4: sd->sensors->font_size = "x-large"; break;
panel-plugin/sensors-plugin.c:        default: sd->sensors->font_size = "medium";

So g_free is only correct when the font size is read from the config file. The rest of the time it will make the plugin crash. Duping these static strings should fix that.
Thanks!