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## Environment
**Liquibase Version**: 4.0.0

## Description
I am getting an NPE when I create a new FileSystemResourceAccessor(rootPath), with the following stack trace:

```
java.lang.NullPointerException
        at liquibase.Scope.getLog(Scope.java:278)
        at liquibase.resource.FileSystemResourceAccessor.addRootPath(FileSystemResourceAccessor.java:54)
        at liquibase.resource.FileSystemResourceAccessor.<init>(FileSystemResourceAccessor.java:36)
```

Looking at the Liquibase code, I see that SmartMap#get defines its result as a nullable, however, the Scope class does not check for null in this and other places.

## Steps To Reproduce
I'm running tests in my project, using liquibase-core:
Created this issue for tracking purposes, where you can also find the full stack trace: https://github.com/Treehopper/liquibase-lsp/issues/17

## Additional Context
In my project, I have to run scripts asynchronously. This, and the fact that the problem only occurs occasionally hints at a race-condition.


for (Map.Entry<String, Object> entry : scopeValues.entrySet()) {
                values.put(entry.getKey(), entry.getValue());
            }
        }
    }
    /**
     * Returns the parent scope to this scope. Returns null if this is a root scope.
     */
    public Scope getParent() {
        return parent;
    }
    /**
     * Creates a new scope that is a child of this scope.
     */
    public static void child(Map<String, Object> scopeValues, ScopedRunner runner) throws Exception {
        child((LiquibaseListener) null, scopeValues, runner);
    }
    /**
     * Creates a new scope that is a child of this scope.
     */
    public static <ReturnType> ReturnType child(Map<String, Object> scopeValues, ScopedRunnerWithReturn<ReturnType> runner) throws Exception {
        return child(null, scopeValues, runner);
    }
    /**
     * Creates a new child scope that includes the given {@link LiquibaseListener}.
     * You cannot unassign a listener, they simply fall out of scope when the Scope does.
     *
     * @see #getListeners(Class)
     */
    public static void child(LiquibaseListener listener, ScopedRunner runner) throws Exception {