My German Apple Cake sank in the middle and I don't know how to fix it I baked a German Apple Cake with a basic streusel on top, it rose on the edges and fell in the center. I'm wondering if it was because the streusel didn't reach the edges of the cake, or possibly because I put it on too early. 

I followed the recipe I found on this website.

2 eggs
1 cup vegetable oil
2 cups white sugar
2 teaspoons ground cinnamon
1/2 teaspoon salt
1 teaspoon vanilla extract
2 cups all-purpose flour
1 teaspoon baking soda
4 cups apples - peeled, cored and diced


Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 9x13 inch cake pan.
In a mixing bowl; beat oil and eggs with an electric mixer until creamy. Add the sugar and vanilla and beat well.  
Combine the flour, salt, baking soda, and ground cinnamon together in a bowl. Slowly add this mixture to the egg mixture and mix until combined. The batter will be very thick. Fold in the apples by hand using a wooden spoon. Spread batter into the prepared pan.  
Bake at 350 degrees F (175 degrees C) for 45 minutes or until cake tests done. Let cake cool on a wire rack. Once cake is cool serve with a dusting of confectioners' sugar or with a Cream Cheese Frosting.
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!