title: Is there a way to fix Invalid field path (``).issues arrises when accessing Firebase from Android App?
body: <p>I had android app working fine with Firebase Database. Recently I start getting java.lang.IllegalArgumentException: Invalid field path (``). Paths must not be empty, begin with '.', end with '.', or contain '..' issue. The issue happens if I want to access or write any Collection or Document. In fact it happens every time  I am accessing Firebase from App for read or write operations. The issue is not caused due to blank or null keys of document which was normally found when searching for solution.</p>
  
  <p>I thought issue was of older version being in used and hence upgraded to latest Gradle version but still problem is continuing. I also checked if document keys are not null or blank since on searching found issue could be due to blank keys as well. Dint find such cause as well. Further i also observed the problem comes in accessing any collection or document of the database. That makes me thinking it is not coding issue but possibly more of generic Firebase issue that occurs every time during read or write operations performed. Thoroughly checked documentation but dint get clue for resolving the issue. Hence reverted to seek expert help from you guys.</p>
  
  <p>My current setting as follows :-</p>
  
  <pre><code> apply plugin: 'com.android.application'
    android {
            compileSdkVersion 28
    defaultConfig {
      applicationId "com.support.android.designlibdemo"
      minSdkVersion 15
      targetSdkVersion 28
      versionCode 1
      versionName "1.0"
      multiDexEnabled true
      javaCompileOptions {
          annotationProcessorOptions {
              arguments = 
         ["room.schemaLocation":"$projectDir/schemas".toString()]
          }
      }
     }
     buildTypes {
      release {
          minifyEnabled false
          proguardFiles getDefaultProguardFile('proguard-android.txt'), 
       'proguard-rules.pro'}
       }
       dataBinding {
      enabled = true}
  
     compileOptions {
      targetCompatibility 1.8
      sourceCompatibility 1.8
      }
   }
  
   dependencies {
    //noinspection GradleCompatible
    implementation 'com.google.android.material:material:1.0.0'
    implementation 'androidx.cardview:cardview:1.0.0'
    implementation 'com.github.bumptech.glide:glide:4.6.1'
    implementation 'de.hdodenhof:circleimageview:2.2.0'
    // Have used to resolve errors of ViewModelProviders two lines
     implementation 'androidx.lifecycle:lifecycle-extensions:2.0.0'
    implementation 'com.google.firebase:firebase-database:16.0.4'
     annotationProcessor 'androidx.lifecycle:lifecycle-compiler:2.0.0'
  
   // ViewModel and LiveData
   // implementation "android.arch.lifecycle:extensions:1.0.0"
   //  annotationProcessor "android.arch.lifecycle:compiler:1.0.0"
  
   // Room
   implementation 'androidx.room:room-runtime:2.0.0'
   annotationProcessor 'androidx.room:room-compiler:2.0.0'
  
   // Paging
   //   implementation "android.arch.paging:runtime:1.0.0-alpha6-1"
   //Firebase
   // implementation "com.google.firebase:firebase-core:16.0.4"
   // implementation 'com.google.firebase:firebase-database:19.2.0'
   implementation 'com.google.firebase:firebase-firestore:21.2.1'
   //Firestore Storage
   implementation 'com.google.firebase:firebase-storage:19.1.0'
   implementation 'androidx.multidex:multidex:2.0.0'
   // Image Liberary Fresco from Open Source Facebook developer
   implementation 'com.facebook.fresco:fresco:1.10.0'
   }
   //Firebase
   apply plugin: "com.google.gms.google-services"
   buildscript {
    repositories {
      maven { url 'https://maven.google.com' }
      jcenter()
      google()
    }
   dependencies {
      classpath 'com.android.tools.build:gradle:3.5.1'
      //FIrebase
      classpath 'com.google.gms:google-services:4.3.2'
  }
  }
  
  allprojects {
  repositories {
      maven { url 'https://maven.google.com' }
      jcenter()
      google()
  }
  </code></pre>
  
  <p>}</p>
  
  <p>The code used for accessing FireBase is </p>
  
  <pre><code>   FirebaseFirestore
     db = FirebaseFirestore.getInstance();
      FirebaseFirestoreSettings settings = new 
      FirebaseFirestoreSettings.Builder()
              .build();
      db.setFirestoreSettings(settings);
  
     DocumentReference docref = 
        db.collection("Forms").document("Carpenter");
     docref.get().addOnCompleteListener(new 
     OnCompleteListener&lt;DocumentSnapshot&gt;() {
         @Override
         public void onComplete(@NonNull Task&lt;DocumentSnapshot&gt; task) {
            .........
  
             } 
         }
      });
  </code></pre>
  
  <p>Error Message </p>
  
  <pre><code>  2019-10-29 09:58:58.077 4417-4417/com.support.android.designlibdemo 
    E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.support.android.designlibdemo, PID: 4417
    java.lang.RuntimeException: Internal error in Cloud Firestore (21.2.0).
    at com.google.firebase.firestore.util.AsyncQueue.lambda$panic$3 
    (com.google.firebase:firebase-firestore@@21.2.1:529)
      at com.google.firebase.firestore.util.AsyncQueue$$Lambda$3.run(Unknown 
    Source:2)
      at android.os.Handler.handleCallback(Handler.java:790)
      at android.os.Handler.dispatchMessage(Handler.java:99)
      at android.os.Looper.loop(Looper.java:164)
      at android.app.ActivityThread.main(ActivityThread.java:6494)
      at java.lang.reflect.Method.invoke(Native Method)
      at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run
      (RuntimeInit.java:438)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
      Caused by: java.lang.RuntimeException: 
      java.lang.IllegalArgumentException: Invalid field path (``). Paths 
      must not be empty, begin with '.', end with '.', or contain '..'
      at com.google.firebase.firestore.util.
      AsyncQueue$SynchronizedShutdownAwareExecutor.
      lambda$executeAndReportResult$1(com.google.firebase:firebase- 
      firestore@@21.2.1:320)
      at com.google.firebase.firestore.util.
      AsyncQueue$SynchronizedShutdownAwareExecutor$$Lambda$2.run(Unknown 
      Source:4)
      at java.util.concurrent.Executors$RunnableAdapter.call
      (Executors.java:457)
      at java.util.concurrent.FutureTask.run(FutureTask.java:266)
      at java.util.concurrent.ScheduledThreadPoolExecutor$
      ScheduledFutureTask.run
      (ScheduledThreadPoolExecutor.java:301)
      at java.util.concurrent.ThreadPoolExecutor.runWorker
       (ThreadPoolExecutor.java:1162)
      at java.util.concurrent.ThreadPoolExecutor$Worker.run
      (ThreadPoolExecutor.java:636)
      at com.google.firebase.firestore.util.
      AsyncQueueSynchronizedShutdownAwareExecutor$DelayedStartFactory.
     run(com.google.firebase:firebase-firestore@@21.2.1:224)
      at java.lang.Thread.run(Thread.java:764)
  </code></pre>