My German Apple Cake sank in the middle and I don't know how to fix it
There are a few reasons a cake might sink. First, you may have added too much baking soda or the baking soda may have been old. Or, you may have allowed your batter to sit for too long. Both of these would contribute to your cake over rising and then collapsing. 
 Otherwise, most cake-sink cases have a lot to do with temperature. It's possible your oven doesn't reach the temperature it's set at (can be solved with  handy thermometer) or you opened the oven too many times to check on your cake. 
 If you're concerned about the streusel, try to apply it in a thin even layer, although I've never had a problem with it being so heavy the cake won't rise. Hope this helps!