compose_box [nfc]: Call setState post frame in _SendButtonState._hasErrorsChanged

This will prevent throwing the "setState() or markNeedsBuild() called
during build" assertion when a recipient is disabled in a DM conversation
with compose box being populated with some text, in the next commit.

Author: sm-sayedi

lib/widgets/compose_box.dart

@@ -679,8 +679,10 @@ class _SendButton extends StatefulWidget {
 
 class _SendButtonState extends State<_SendButton> {
   void _hasErrorsChanged() {
-    setState(() {
-      // Update disabled/non-disabled state
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      setState(() {
+        // Update disabled/non-disabled state
+      });
     });
   }