Does laughter invalidate wudu' (ablution) if one is not praying but preparing to pray?

There is no disagreement that laughter outside of prayer does not invalidate ablution. Scholars have agreed that laughter (guffawing) during prayer nullifies the prayer itself.

As for laughter invalidating ablution during prayer, the majority view is that it does not invalidate it due to the absence of evidence. It has been authentically narrated from Jabir ibn Abdullah (may Allah be pleased with him) that he said: "If one laughs during prayer, he repeats the prayer, but he does not repeat the ablution." No differing opinion has been authentically narrated from any Companion. The hadiths reported concerning laughter invalidating ablution are weak by the consensus of the hadith scholars.

In summary, laughter does not invalidate ablution, whether it occurs inside or outside of prayer.