cycleway: fix bug in setting the the sidewalk value (fixes #740)

When the user selected "cyclists may use sidewalk" for one side and "cyclists must use sidewalk" for the other, the application crashed.

app/src/main/java/de/westnordost/streetcomplete/quests/bikeway/AddCycleway.java

@@ -58,12 +58,32 @@ public class AddCycleway implements OsmElementQuestType
 			}
 		}
 
+		applySidewalkAnswerTo(cyclewayLeft, cyclewayRight, changes);
+
 		if(answer.getBoolean(AddCyclewayForm.IS_ONEWAY_NOT_FOR_CYCLISTS))
 		{
 			changes.addOrModify("oneway:bicycle", "no");
 		}
 	}
 
+	private void applySidewalkAnswerTo(Cycleway cyclewayLeft, Cycleway cyclewayRight,
+									   StringMapChangesBuilder changes)
+	{
+		boolean hasSidewalkLeft = cyclewayLeft != null && cyclewayLeft.isOnSidewalk();
+		boolean hasSidewalkRight = cyclewayRight != null && cyclewayRight.isOnSidewalk();
+
+		Side side;
+		if(hasSidewalkLeft && hasSidewalkRight)	side = Side.BOTH;
+		else if(hasSidewalkLeft)				side = Side.LEFT;
+		else if(hasSidewalkRight)				side = Side.RIGHT;
+		else									side = null;
+
+		if(side != null)
+		{
+			changes.addOrModify("sidewalk", side.value);
+		}
+	}
+
 	private enum Side
 	{
 		LEFT("left"), RIGHT("right"), BOTH("both");
@@ -109,12 +129,10 @@ private void applyCyclewayAnswerTo(Cycleway cycleway, Side side, int dir, String
 				// https://wiki.openstreetmap.org/wiki/File:Z240GemeinsamerGehundRadweg.jpeg
 				changes.add(cyclewayKey, "track");
 				changes.add(cyclewayKey + ":segregated", "no");
-				changes.addOrModify("sidewalk", side.value);
 				break;
 			case SIDEWALK_OK:
 				// https://wiki.openstreetmap.org/wiki/File:Z239Z1022-10GehwegRadfahrerFrei.jpeg
 				changes.add(cyclewayKey, "no");
-				changes.addOrModify("sidewalk", side.value);
 				changes.add("sidewalk:" + side.value + ":bicycle", "yes");
 				break;
 			case SHARED:

app/src/main/java/de/westnordost/streetcomplete/quests/bikeway/Cycleway.java

@@ -29,4 +29,9 @@ public int getIconResId(boolean isLeftHandTraffic)
 	{
 		return isLeftHandTraffic ? iconResIdLeft : iconResId;
 	}
+
+	public boolean isOnSidewalk()
+	{
+		return this == SIDEWALK || this == SIDEWALK_OK;
+	}
 }