Exception when deleting code block identifier

[fxha]

Description

An exception occurs when deleting the code block identifier and pressing multiple backspace after.

• Can you reproduce the issue?

Steps to reproduce

1. Create a code block like below (without \)
2. Remove cpp and press multiple backspaces when your removed cpp

foo

\```cpp
foo
bar
\```

baz

Stack Trace:

exportMarkdown.js:228 Uncaught TypeError: Cannot read property 'children' of undefined
    at ds.normalizeCodeBlock (exportMarkdown.js:228)
    at ds.translateBlocks2Markdown (exportMarkdown.js:146)
    at ds.generate (exportMarkdown.js:31)
    at rr.getMarkdown (index.js:123)
    at rr.Br.dispatchChange (index.js:100)
    at keyboard.js:89

Versions

• Mark Text: v0.15.0 (release draft) - commit e2bac53
• Operating system: all