Don't crash on an index cascade section followed by a method chain. (#1556)

Don't crash on an index cascade section followed by a method chain.

I found this weird corner case in the wild when formatting a large
corpus.

Author: munificent

lib/src/front_end/chain_builder.dart

@@ -327,8 +327,13 @@ class ChainBuilder {
           });
         });
 
-      case IndexExpression():
-        _unwrapPostfix(expression.target!, (target) {
+      case IndexExpression(:var target?):
+        // We check for a non-null target because the target may be `null` if
+        // the chain we are building is itself in a cascade section that begins
+        // with an index expression like:
+        //
+        //     object..[index].chain();
+        _unwrapPostfix(target, (target) {
           return _visitor.pieces.build(() {
             _visitor.pieces.add(target);
             _visitor.writeIndexExpression(expression);

test/tall/invocation/cascade_mixed.stmt

@@ -36,4 +36,8 @@ object..cascade()!..cascade()[index]..cascade()(arg);
 object
   ..cascade()!
   ..cascade()[index]
-  ..cascade()(arg);
+  ..cascade()(arg);
+>>> Chain with index target.
+object..[index].method();
+<<<
+object..[index].method();