Fix #499

mastodon/src/main/java/org/joinmastodon/android/fragments/ComposeFragment.java

@@ -544,6 +544,8 @@ public class ComposeFragment extends MastodonToolbarFragment implements OnBackPr
 
 			@Override
 			public void afterTextChanged(Editable s){
+				if(s.length()==0)
+					return;
 				int start=lastChangeStart;
 				int count=lastChangeCount;
 				// offset one char back to catch an already typed '@' or '#' or ':'