Repl Allows One Command to Execute and Subsequent Commands Fail

**Description**

When I use the repl, I can run one command and that's it. After that I get an error.

**Steps to reproduce the issue:**
1. run repl
2. run 2 + 2 (works)
3. run 2 + 1 (error)

**Received results:**

```
cayley> 2 + 2

=> 4
-----------
1 Result
Elapsed time: 1.95 ms

cayley> 2+1

Error: iterator closed at :1:1(0)
cayley>
```

**Expected results:**

It should allow me to run multiple commands

**Output of `cayley version` or commit hash:**

```
ERROR: logging before flag.Parse: E1014 21:20:53.997357 97410 cayley.go:142] The assets flag is deprecated as assets are provided as part of the binary.
Cayley version: 0.7.7
Git commit hash: dcf764f
```

**Environment details:**

Backend database: `bolt` version unknown

Start at Cayley's REPL entry point and reproduce the reported sequence with version 0.7.7 and the bolt backend: run 2 + 2, then 2 + 1. Trace why the iterator is closed after the first command, and verify that multiple commands complete without the reported error.

Written by the indexing model from the issue text.