[client channel] if picker is null, fail call instead of crashing

src/core/client_channel/client_channel_filter.cc

@@ -2894,6 +2894,15 @@ ClientChannelFilter::LoadBalancedCall::PickSubchannel(bool was_queued) {
     set_picker(chand_->picker_);
   }
   while (true) {
+    // TODO(roth): Fix race condition in channel_idle filter and any
+    // other possible causes of this.
+    if (pickers.back() == nullptr) {
+      if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_lb_call_trace)) {
+        gpr_log(GPR_ERROR, "chand=%p lb_call=%p: picker is null, failing call",
+                chand_, this);
+      }
+      return absl::InternalError("picker is null -- shouldn't happen");
+    }
     // Do pick.
     if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_lb_call_trace)) {
       gpr_log(GPR_INFO, "chand=%p lb_call=%p: performing pick with picker=%p",