Bug 874754 - Suppress nsIEditor.removeEditorObserver exception. r=fabrice

Based on patch from Yuan Xulei <xyuan@mozilla.com>

--HG--
extra : rebase_source : 77b563eaa784d42cdfd20a3aba1f5b928a53aaa6

b2g/chrome/content/forms.js

@@ -235,7 +235,13 @@ let FormAssistant = {
 
     this._documentEncoder = null;
     if (this._editor) {
-      this._editor.removeEditorObserver(this);
+      // When the nsIFrame of the input element is reconstructed by
+      // CSS restyling, the editor observers are removed. Catch
+      // [nsIEditor.removeEditorObserver] failure exception if that
+      // happens.
+      try {
+        this._editor.removeEditorObserver(this);
+      } catch (e) {}
       this._editor = null;
     }