make the `get` API of editor clearer

src/app.js

@@ -896,7 +896,9 @@ Please make a backup of your contracts and start using http://remix.ethereum.org
       return
     }
     var input = editor.get(currentFile)
-
+    if (!input) {
+      return
+    }
     // if there's no change, don't do anything
     if (input === previousInput) {
       return

src/app/editor/editor.js

@@ -169,13 +169,29 @@ function Editor (opts = {}) {
     switchSession(path)
   }
 
+  /**
+    * returns the content of the specified session @arg path
+    * if @arg path is not provided, the content of the current editing session is returned
+    *
+    * @param {String} path    - path of th file in edition
+    * @return {String} content of the file referenced by @arg path
+    */
   this.get = function (path) {
-    if (currentSession === path) {
+    if (!path || currentSession === path) {
       return editor.getValue()
+    } else if (sessions[path]) {
+      sessions[path].getValue()
     }
   }
 
-  this.current = function (path) {
+  /**
+    * returns the path of the currently editing file
+    * returns `undefined` if no session is being editer
+    *
+    * @param {String} path    - path of th file in edition
+    * @return {String} content of the file referenced by @arg path
+    */
+  this.current = function () {
     if (editor.getSession() === emptySession) {
       return
     }

src/app/files/file-explorer.js

@@ -28,7 +28,7 @@ function fileExplorer (appAPI, files) {
   }
 
   this.files.event.register('fileExternallyChanged', (path, file) => {
-    if (appAPI.config.get('currentFile') === path && appAPI.currentContent() !== file.content) {
+    if (appAPI.config.get('currentFile') === path && appAPI.currentContent() && appAPI.currentContent() !== file.content) {
       modalDialog(path + ' changed', remixdDialog(),
         {
           label: 'Keep the content displayed in Remix',

src/app/files/fileManager.js

@@ -174,6 +174,7 @@ class FileManager {
     var currentFile = this.opt.config.get('currentFile')
     if (currentFile && this.opt.editor.current()) {
       var input = this.opt.editor.get(currentFile)
+      if (input) {
         var provider = this.fileProviderOf(currentFile)
         if (provider) {
           provider.set(currentFile, input)
@@ -182,6 +183,7 @@ class FileManager {
         }  
       }
     }
+  }
 }
 
 module.exports = FileManager