private HashMap<String, Item> getFileIdMap() {
		if (fileIdMap == null) {
			fileIdMap = new HashMap<>();
		}
		return fileIdMap;
	}
Construct existing indexed filesystem with an empty index.
This can be used to prepare for rebuilding the filesystem index.
@param rootPath
@throws IOException
Ghidra/Framework/FileSystem/src/main/java/ghidra/framework/store/local/IndexedV1LocalFileSystem.java