Issue with "bb theme-build --sourcemaps" not working on Windows #143

Steps to reproduce
Execute the tool in the root theme folder
example path: "D:\dev\alder_new\statics\collection\collection-aldermore\theme-asset-finance"
Executing: "bb theme-build --sourcemaps"

Error log:
TypeError: Path must be a string. Received undefined
at assertPath (path.js:7:11)
at Object.dirname (path.js:697:5)
at C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\clean-css\lib\utils\input-source-map-tracker.js:191:56
at Array.forEach (native)
at trackContentSources (C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\clean-css\lib\utils\input-source-map-tracker.js:188:20)
at InputSourceMapStore.trackLoaded (C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\clean-css\lib\utils\input-source-map-tracker.js:255:3)
at fromString (C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\clean-css\lib\utils\input-source-map-tracker.js:32:8)
at InputSourceMapStore.track (C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\clean-css\lib\utils\input-source-map-tracker.js:236:5)
at C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\clean-css\lib\clean.js:145:44
at CleanCSS.minify (C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\clean-css\lib\clean.js:127:42)
at C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\gulp-minify-css\index.js:39:35
at vinylBufferStream (C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\vinyl-bufferstream\index.js:50:5)
at Transform.modifyContents as _transform
at Transform._read (C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\gulp-minify-css\node_modules\readable-stream\lib_stream_transform.js:172:10)

at Transform._write (C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\gulp-minify-css\node_modules\readable-stream\lib\_stream_transform.js:160:12

)
at doWrite (C:\Users\nikola\AppData\Roaming\npm\node_modules\bb-cli\node_modules\gulp-minify-css\node_modules\readable-stream\lib_stream_writable.js:333:12)