Covering the last two dirs in methods to prevent crashing the app in case it's null

Fixes #34

Author: Ron Radtke

android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilFS.java

@@ -250,8 +250,8 @@ else if(resolved == null) {
     static Map<String, Object> getSystemfolders(ReactApplicationContext ctx) {
         Map<String, Object> res = new HashMap<>();
 
-        res.put("DocumentDir", ctx.getFilesDir().getAbsolutePath());
-        res.put("CacheDir", ctx.getCacheDir().getAbsolutePath());
+        res.put("DocumentDir", getFilesDirPath(ctx));
+        res.put("CacheDir", getCacheDirPath(ctx));
         res.put("DCIMDir", getExternalFilesDirPath(ctx, Environment.DIRECTORY_DCIM));
         res.put("PictureDir", getExternalFilesDirPath(ctx, Environment.DIRECTORY_PICTURES));
         res.put("MusicDir", getExternalFilesDirPath(ctx, Environment.DIRECTORY_MUSIC));
@@ -282,6 +282,18 @@ static String getExternalFilesDirPath(ReactApplicationContext ctx, String type)
         return "";
     }
 
+    static String getFilesDirPath(ReactApplicationContext ctx) {
+        File dir = ctx.getFilesDir();
+        if (dir != null) return dir.getAbsolutePath();
+        return "";
+    }
+
+    static String getCacheDirPath(ReactApplicationContext ctx) {
+        File dir = ctx.getCacheDir();
+        if (dir != null) return dir.getAbsolutePath();
+        return "";
+    }
+
     static public void getSDCardDir(ReactApplicationContext ctx, Promise promise) {
         if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
             try {