Investigate "Attempted makeEl before iframe was initialized"

[ianb]

In [this report(https://sentry.prod.mozaws.net/operations/screenshots-addon-prod/issues/616028/):

resource://screenshots-addon/selector/ui.js in makeEl at line 52:13
resource://screenshots-addon/selector/ui.js in display at line 611:19
resource://screenshots-addon/selector/uicontrol.js in mousemove at line 384:7
resource://screenshots-addon/selector/uicontrol.js in addHandlers/</fn< at line 884:18
resource://screenshots-addon/catcher.js in watchFunction/this.catcher at line 54:161
resource://screenshots-addon/selector/uicontrol.js in EventListener.handleEvent*installHandlersOnDocument at line 900:7
resource://screenshots-addon/selector/ui.js in display/this.ui</this.element.onload< at line 247:13
resource://screenshots-addon/catcher.js in watchFunction/this.catcher at line 54:16
resource://screenshots-addon/selector/ui.js in EventHandlerNonNull*display/this.ui< at line 222:33
resource://screenshots-addon/selector/ui.js in display at line 209:14
resource://screenshots-addon/selector/ui.js in display/this.ui< at line 329:21
resource://screenshots-addon/selector/ui.js in promise callback*display at line 328:14
resource://screenshots-addon/selector/uicontrol.js in start at line 374:20
resource://screenshots-addon/selector/uicontrol.js in setState at line 192:7
resource://screenshots-addon/selector/uicontrol.js in exports.activate at line 835:7
resource://screenshots-addon/catcher.js in watchFunction/this.catcher at line 54:16
resource://screenshots-addon/selector/uicontrol.js in this.uicontrol< at line 941:3
resource://screenshots-addon/selector/uicontrol.js at line 6:19

[chenba]

(I'm not familiar with the code from when the issue was initially filed, but my comment probably still apply.) All three iframes are initialized in the same promise chain when the UI state is set to crosshair, which is the initial state when there is no onboarding. But if a user manages to click on the save full page or save visible button before the preview frame is initialized, the user would then encounter this error.