Add exception handling to MD5 code

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobReq.java

     }
 
     public static String getMD5(String input) {
-        MessageDigest md = MessageDigest.getInstance("MD5");
-        md.update(input.getBytes());
-        byte[] digest = md.digest();
-        
-        StringBuffer sb = new StringBuffer();
-        
-        for (byte b : digest) {
-            sb.append(String.format("%02x", b & 0xff))
+        String result = null;
+
+        try {
+            MessageDigest md = MessageDigest.getInstance("MD5");
+            md.update(input.getBytes());
+            byte[] digest = md.digest();
+            
+            StringBuffer sb = new StringBuffer();
+            
+            for (byte b : digest) {
+                sb.append(String.format("%02x", b & 0xff));
+            }
+
+            result = sb.toString();
+        } catch(Exception ex) {
+            ex.printStackTrace();
         }
 
-        return sb.toString();
+        return result;
     }
 
     @Override
 
         }
 
-        String key = this.taskId;
+        String cacheKey = this.taskId;
         if (this.options.key != null) {
-            key = RNFetchBlobReq.getMD5(this.options.key);
+            cacheKey = RNFetchBlobReq.getMD5(this.options.key);
+            if (cacheKey == null) {
+                cacheKey = this.taskId;
+            }
 
-            File file = new File(RNFetchBlobFileHandler.getFilePath(ctx, taskId, key, this.options))
+            File file = new File(RNFetchBlobFileHandler.getFilePath(ctx, taskId, cacheKey, this.options));
             if (file.exists()) {
                callback.invoke(null, file.getAbsolutePath());
                return;
 
             // create handler
             if(options.fileCache || options.path != null) {
-                handler = new RNFetchBlobFileHandler(ctx, taskId, key, options, callback);
+                handler = new RNFetchBlobFileHandler(ctx, taskId, cacheKey, options, callback);
                 // if path format invalid, throw error
                 if (!((RNFetchBlobFileHandler)handler).isValid) {
                     callback.invoke("RNFetchBlob fetch error, configuration path `"+ options.path  +"` is not a valid path.");