Fix crash when applying theme early in app lifecycle

Fixes #688

internal/app/theme.go

@@ -8,6 +8,9 @@ import (
 // ApplyThemeTo ensures that the specified canvasobject and all widgets and themeable objects will
 // be updated for the current theme.
 func ApplyThemeTo(content fyne.CanvasObject, canv fyne.Canvas) {
+	if content == nil {
+		return
+	}
 	if wid, ok := content.(fyne.Widget); ok {
 		for _, o := range cache.Renderer(wid).Objects() {
 			ApplyThemeTo(o, canv)

internal/app/theme_test.go

@@ -0,0 +1,14 @@
+package app
+
+import (
+	"testing"
+
+	"fyne.io/fyne/test"
+)
+
+func TestApplySettings_BeforeContentSet(t *testing.T) {
+	a := test.NewApp()
+	_ = a.NewWindow("NoContent")
+
+	ApplySettings(a.Settings(), a)
+}