A number of users of 2.22b have experienced this error

At the moment it is unclear if it is a bug, or whether it is simply caused by users entering zero values by mistake.

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This error will occur if a zero value is entered in the cell quantity field.

telega made the following comment in relation to this problem.

Check what values you have specified for thumbnails "Quantity" option in Preferences > General. I think you might put a zero value there. Currently I don't have another idea of why this might happen.

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The first thing that can be done is to try and alter manually the quantity value in Preferences General.

You can either access Preferences by right clicking on Fast Dial,or alternatively going to Add-ons and selecting Options in Fast Dial.

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If you find you cannot change the Quantity value in Preferences, then please try clicking on the reset defaults in Preferences/General and see if that clears the null value:

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If for some reason that still does not work please try the following.

Resetting Fast Dial settings in about:config

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So what we are trying to do is to remove the zero value that has somehow got into the cell size field.

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Please check to see if any of the above has the desired effect.

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Regarding uninstalling and reinstalling, it is not at all clear why this should resolve this particular problem.
So it is not highly recommmended as a solution to this problem.
But it did work for one user.

Might be worth trying uninstalling and reinstalling a couple of times if none of the above procedures have any effect.

Many thanks