Incorporate a suitable thickening agent, such as flour, in your fruit filling. the flour helps absorb excess moisture released by the fruit as it bakes, reducing the chances of a soggy texture. Follow the recipe guidelines for the recommended amount of thickener based on the quantity of fruit you're using. If your fruit is particularly juicy or releases a lot of liquid while macerating, drain off some of the excess liquid before assembling the fruit crisp. This can be done by using a slotted spoon or transferring the fruit to a colander and allowing it to drain for a few minutes. I doubt that will be an issue with this rhubarb filling, though.