fix: check for null variable when excludeNotExported is true

[raymondfeng]

The PR fixes the TypeError when excludeNotExported option is set to true against the following TS file:

export const x = 5;

export function add(x: number, y: number) {
    return x + y;
}

// Add a local var/function to make sure typedoc won't choke
// if excludeNotExported is true
let localVar = 'local';

function times(x: number, y: number) {
    return x * y;
}

The stack trace was:

TypeError: Cannot read property 'setFlag' of null
      at VariableConverter.convert (node_modules/typedoc/dist/lib/converter/nodes/variable.js:79:30)
      at Converter.convertNode (node_modules/typedoc/dist/lib/converter/converter.js:127:53)
      at node_modules/typedoc/dist/lib/converter/nodes/variable-statement.js:39:33
      at Array.forEach (<anonymous>)
      at VariableStatementConverter.convert (node_modules/typedoc/dist/lib/converter/nodes/variable-statement.js:34:47)
      at Converter.convertNode (node_modules/typedoc/dist/lib/converter/converter.js:127:53)
      at node_modules/typedoc/dist/lib/converter/nodes/block.js:85:29
      at Array.forEach (<anonymous>)
      at BlockConverter.convertStatements (node_modules/typedoc/dist/lib/converter/nodes/block.js:84:26)
      at node_modules/typedoc/dist/lib/converter/nodes/block.js:62:27
      at Context.withScope (node_modules/typedoc/dist/lib/converter/context.js:107:9)
      at node_modules/typedoc/dist/lib/converter/nodes/block.js:61:25
      at Context.withSourceFile (node_modules/typedoc/dist/lib/converter/context.js:87:9)
      at BlockConverter.convertSourceFile (node_modules/typedoc/dist/lib/converter/nodes/block.js:58:17)
      at BlockConverter.convert (node_modules/typedoc/dist/lib/converter/nodes/block.js:48:18)
      at Converter.convertNode (node_modules/typedoc/dist/lib/converter/converter.js:127:53)
      at node_modules/typedoc/dist/lib/converter/converter.js:155:19
      at Array.forEach (<anonymous>)
      at Converter.compile (node_modules/typedoc/dist/lib/converter/converter.js:154:34)
      at Converter.convert (node_modules/typedoc/dist/lib/converter/converter.js:110:27)

[aciccarello]

Thanks! I was just working on that but this is great!

[raymondfeng]

@aciccarello Thank you for the prompt acceptance. I look forward to a new typedoc release soon.

[aciccarello]

Thanks for a prompt PR! I'm going to hold off on a patch release for now in case there are any other issues from the last release. I also want to improve some of the test flakiness so the release process is easier.

[EvanDarwin]

Awesome! Thanks for the quick response guys! 👍

[patrickhulce]

Awesome work! I was just visiting the repo to find this :) any word on when a 0.10.1 will go out with this fix @aciccarello?

[dupski]

I can confirm that latest master fixes this issue for me too @aciccarello