If anyone is getting this error after upgrading, it's because inside the file default.conf (~/.config/codeblocks on linux) contains a "color" tag with a property "null".
You should change it from this:
<HIGHLIGHT_CARET_LINE_COLOUR>
<colour r="24" g="24" b="24" null="true" />
</HIGHLIGHT_CARET_LINE_COLOUR>
To this:
<HIGHLIGHT_CARET_LINE_COLOUR>
<colour r="24" g="24" b="24" />
</HIGHLIGHT_CARET_LINE_COLOUR>
And the error goes away.