1. Go here: https://jsfiddle.net/u6z2a5s4/
2. Draw a polygon
3. Enter direct_select mode by clicking on the polygon twice.
4. Click the combine_features or uncombine_features button, and experience that there are errors in the console:

object_to_mode.js:61 Uncaught TypeError: i.onCombineFeatures is not a function
    at Object.combineFeatures (object_to_mode.js:61:14)

I would expect the behavior of the buttons to be similar to in simple_select mode when having selected only one polygon: No effect, yet without uncaught type errors.

probably the safest is to solve by checking for the function before calling it:

combineFeatures() { 
-  mode.onCombineFeatures(state); 
+ if (mode.onCombineFeatures instanceof Function ) {
+    mode.onCombineFeatures(state); 
+ }
 }, 
 uncombineFeatures() { 
-  mode.onUncombineFeatures(state);
+ if (mode.onUncombineFeatures instanceof Function ) {
+    mode.onUncombineFeatures(state); 
+  }
 }

Further, I would also have expected such a TypeError to be detected at compilation time, since both onCombineFeature and onUncombineFeature are well defined as optional in the DrawCustomMode interface