Fix delete button not working

lib/models/documents/nodes/container.dart

@@ -59,7 +59,12 @@ abstract class Container<T extends Node> extends Node {
   }
 
   ChildQuery queryChild(int offset, bool inclusive) {
-    assert(offset >= 0 && offset <= length);
+    if (offset < 0) {
+      throw ('Offset cannot be negative');
+    }
+    if (offset > length) {
+      throw ('offset $offset > $length');
+    }
 
     for (Node node in children) {
       int len = node.length;

lib/models/documents/nodes/line.dart

@@ -128,7 +128,7 @@ class Line extends Container<Leaf> {
       super.delete(index, local);
     }
 
-    int remain = length - local;
+    int remain = len - local;
     if (remain > 0) {
       assert(nextLine != null);
       nextLine.delete(0, remain);

lib/widgets/controller.dart

@@ -34,7 +34,12 @@ class QuillController extends ChangeNotifier {
 
     Delta delta;
     if (len > 0 || data is! String || (data as String).isNotEmpty) {
-      delta = document.replace(index, len, data);
+      try {
+        delta = document.replace(index, len, data);
+      } catch (e) {
+        print ('document.replace failed: $e');
+        throw e;
+      }
       if (delta != null &&
           toggledStyle.isNotEmpty &&
           delta.isNotEmpty &&