Stir the sugar and flour together in a mixing bowl. In a separate, large mixing bowl, stir the beaten egg into the chopped rhubarb. Add the sugar/flour mixture. Spoon this rhubarb filling mixture into the unbaked pie shell. To prepare the crumb topping... mix the flour and sugars together in a large bowl. Cut the chilled butter or margarine into small pieces. Using 2 table knives or pastry blender, cut the butter or margarine pieces into the flour and sugar until the mixture looks like coarse crumbs. Sprinkle the crumbly mixture evenly over the rhubarb filling in the pie shell. Bake in a preheated oven at 400º for 10 minutes. Reduce heat to 375º and bake for about 50 more minutes, or until the crust and crumbs are nicely browned and the rhubarb filling is thickened and bubbly.