Bug 734714 - Some prompts broken: 'aDomWin is null' in prompt service r=wesj

mobile/android/components/PromptService.js

@@ -156,7 +156,8 @@ Prompt.prototype = {
     if (aCheckMsg)
       aInputs.push({ type: "checkbox", label: PromptUtils.cleanUpLabel(aCheckMsg), checked: aCheckState.value });
 
-    PromptUtils.fireDialogEvent(this._domWin, "DOMWillOpenModalDialog");
+    if (this._domWin)
+      PromptUtils.fireDialogEvent(this._domWin, "DOMWillOpenModalDialog");
 
     let msg = { type: "Prompt:Show" };
     if (aTitle) msg.title = aTitle;