The sketch editor locks up/ freezes when trying to change some properties of objects. I.e. when trying to change the thickness of a line, text size...

Click on the 'Update' icon (marked 'UPD.', to the left of 'EDIT') in the left-hand-side tools menu. Now the line will in the bottom of the editor will change to contains icons corresponding to the properties you can set on objects, like line thickness, text size, etc. Holding the mouse pointer over an icon will display help text in the top of the editor.

These icons have small squares in their top-left corner. To change a particular property of an object, click on the square to select that property. Be carefull not to click on the icon itself, because this will open a secondary window, which can lock up the editor. The square should be black when selected. You may select multiple properties at the same time.

To change the value of the property, use either the middle and right mousebutton (if you have a three-button mouse) or use shift + left-click and ctrl + left-click on the icon to decrease and increase the value of the property.

When you have set the value you desire, you click on the object you want to update. I.e. click on the line if you want to update its thickness. This will update the line to the new thickness. You may continue to update several objects this way.