--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -2470,81 +2470,76 @@ GeckoDriver.prototype.maximizeWindow = f
   win.maximize()
 };
 
 /**
  * Dismisses a currently displayed tab modal, or returns no such alert if
  * no modal is displayed.
  */
 GeckoDriver.prototype.dismissDialog = function(cmd, resp) {
-  if (!this.dialog) {
-    throw new NoAlertOpenError(
-        "No tab modal was open when attempting to dismiss the dialog");
-  }
+  this._checkIfAlertIsPresent();
 
   let {button0, button1} = this.dialog.ui;
   (button1 ? button1 : button0).click();
   this.dialog = null;
 };
 
 /**
  * Accepts a currently displayed tab modal, or returns no such alert if
  * no modal is displayed.
  */
 GeckoDriver.prototype.acceptDialog = function(cmd, resp) {
-  if (!this.dialog) {
-    throw new NoAlertOpenError(
-        "No tab modal was open when attempting to accept the dialog");
-  }
+  this._checkIfAlertIsPresent();
 
   let {button0} = this.dialog.ui;
   button0.click();
   this.dialog = null;
 };
 
 /**
  * Returns the message shown in a currently displayed modal, or returns a no such
  * alert error if no modal is currently displayed.
  */
 GeckoDriver.prototype.getTextFromDialog = function(cmd, resp) {
-  if (!this.dialog) {
-    throw new NoAlertOpenError(
-        "No tab modal was open when attempting to get the dialog text");
-  }
+  this._checkIfAlertIsPresent();
 
   let {infoBody} = this.dialog.ui;
   resp.body.value = infoBody.textContent;
 };
 
 /**
  * Sends keys to the input field of a currently displayed modal, or
  * returns a no such alert error if no modal is currently displayed. If
  * a tab modal is currently displayed but has no means for text input,
  * an element not visible error is returned.
  */
 GeckoDriver.prototype.sendKeysToDialog = function(cmd, resp) {
-  if (!this.dialog) {
-    throw new NoAlertOpenError(
-        "No tab modal was open when attempting to send keys to a dialog");
-  }
+  this._checkIfAlertIsPresent();
 
   // see toolkit/components/prompts/content/commonDialog.js
   let {loginContainer, loginTextbox} = this.dialog.ui;
   if (loginContainer.hidden) {
     throw new ElementNotVisibleError("This prompt does not accept text input");
   }
 
   let win = this.dialog.window ? this.dialog.window : this.getCurrentWindow();
   event.sendKeysToElement(
       cmd.parameters.value,
       loginTextbox,
       {ignoreVisibility: true},
       win);
 };
 
+GeckoDriver.prototype._checkIfAlertIsPresent = function() {
+  if (!this.dialog || !this.dialog.ui) {
+    throw new NoAlertOpenError(
+        "No tab modal was open when attempting to get the dialog text");
+  }
+};
+
 /**
  * Quits Firefox with the provided flags and tears down the current
  * session.
  */
 GeckoDriver.prototype.quitApplication = function(cmd, resp) {
   if (this.appName != "Firefox") {
     throw new WebDriverError("In app initiated quit only supported in Firefox");
   }