Various crashes fixed

core/src/io/anuke/mindustry/entities/traits/BuilderTrait.java

@@ -202,6 +202,11 @@ public interface BuilderTrait extends Entity{
         //otherwise, update it.
         BuildEntity entity = tile.entity();
 
+        if(entity == null){
+            getPlaceQueue().removeFirst();
+            return;
+        }
+
         //deconstructing is 2x as fast
         if(current.remove){
             entity.deconstruct(unit, core, 2f / entity.buildCost * Timers.delta() * getBuildPower(tile));

core/src/io/anuke/mindustry/entities/units/types/Drone.java

@@ -429,7 +429,7 @@ public class Drone extends FlyingUnit implements BuilderTrait{
     @Override
     public void write(DataOutput data) throws IOException{
         super.write(data);
-        data.writeInt(mineTile == null ? -1 : mineTile.packedPosition());
+        data.writeInt(mineTile == null || !state.is(mine) ? -1 : mineTile.packedPosition());
         data.writeInt(state.is(repair) && target instanceof TileEntity ? ((TileEntity)target).tile.packedPosition() : -1);
         writeBuilding(data);
     }