AttributeError raised for some projects

### Description

Hello

I run ScanCode (3.0.2) through ORT in docker.
I've got that king of error :

```
20:34:09 18:31:10.539 [main] ERROR org.ossreviewtoolkit.scanner.LocalScanner - Could not scan 'Maven:org.jetbrains.kotlin:kotlin-reflect:1.4.0' (68/1712): ScanException: Running '/usr/local/bin/scancode --copyright --license --info --strip-root --timeout 300 --ignore *.ort.yml --ignore HERE_NOTICE --ignore META-INF/DEPENDENCIES --ignore **/test/** --ignore **/tests/** --ignore **/__tests__/** --ignore **/qunit/** --ignore spring-boot-tests/** --ignore doc/** --ignore docs/** --ignore documentation/** --ignore documentations/** --ignore demo/** --ignore **/example/** --ignore **/examples/** --ignore **/sample/** --ignore **/samples/** --ignore changelog.txt --processes 3 /project/reports/scanner/downloads/Maven/org.jetbrains.kotlin/kotlin-reflect/1.4.0 --json-pp /project/reports/scanner/native-scan-results/Maven/org.jetbrains.kotlin/kotlin-reflect/1.4.0/scan-results_ScanCode.json' in '/' failed with exit code 1:
20:34:09 Setup plugins...
20:34:09 Collect file inventory...
20:34:09 ERROR: failed to run pre-scan plugin: ignore:
20:34:09 Traceback (most recent call last):
20:34:09 File "/usr/local/scancode-toolkit-3.0.2/src/scancode/cli.py", line 870, in run_codebase_plugins
20:34:09 plugin.process_codebase(codebase, **kwargs)
20:34:09 File "/usr/local/scancode-toolkit-3.0.2/src/scancode/plugin_ignore.py", line 85, in process_codebase
20:34:09 remove_resource(resource)
20:34:09 File "/usr/local/scancode-toolkit-3.0.2/src/scancode/resource.py", line 753, in remove_resource
20:34:09 for descendant in resource.walk(self, topdown=False):
20:34:09 File "/usr/local/scancode-toolkit-3.0.2/src/scancode/resource.py", line 1117, in walk
20:34:09 for child in self.children(codebase):
20:34:09 File "/usr/local/scancode-toolkit-3.0.2/src/scancode/resource.py", line 1141, in children
20:34:09 return sorted((get_resource(rid) for rid in self.children_rids), key=_sorter)
20:34:09 File "/usr/local/scancode-toolkit-3.0.2/src/scancode/resource.py", line 1139, in
20:34:09 _sorter = lambda r: (r.has_children(), r.name.lower(), r.name)
20:34:09 AttributeError: 'NoneType' object has no attribute 'has_children'
```

it occurs on several projects.

Is it a configuration problem from my part or a scancode issue ?

### How To Reproduce

Take a maven project with that dependency
```

org.jetbrains.kotlin
kotlin-reflect
1.4.0

```

Here the scanCode command line options :
`--copyright --license --info --strip-root --timeout 300 --ignore *.ort.yml --ignore HERE_NOTICE --ignore META-INF/DEPENDENCIES --ignore **/test/** --ignore **/tests/** --ignore **/__tests__/** --ignore **/qunit/** --ignore spring-boot-tests/** --ignore doc/** --ignore docs/** --ignore documentation/** --ignore documentations/** --ignore demo/** --ignore **/example/** --ignore **/examples/** --ignore **/sample/** --ignore **/samples/** --ignore changelog.txt --processes 3`

### System configuration

* What OS are you running on? Linux (docker)
* What version of scancode-toolkit was used to generate the scan file? 3.0.2
* What installation method was used to install/run scancode? install by ORT