Add placeholder clipboard file for mobile driver to avoid crashes if …

…it was accidentally called.

internal/driver/gomobile/clipboard.go

@@ -0,0 +1,22 @@
+package gomobile
+
+import (
+	"fyne.io/fyne"
+)
+
+// Declare conformity with Clipboard interface
+var _ fyne.Clipboard = (*mobileClipboard)(nil)
+
+// mobileClipboard represents the system mobileClipboard
+type mobileClipboard struct {
+}
+
+// Content returns the mobileClipboard content
+func (c *mobileClipboard) Content() string {
+	return "" // TODO implement ticket #414
+}
+
+// SetContent sets the mobileClipboard content
+func (c *mobileClipboard) SetContent(content string) {
+	// TODO implement ticket #414
+}

internal/driver/gomobile/window.go

@@ -171,10 +171,9 @@ func (w *window) Canvas() fyne.Canvas {
 }
 
 func (w *window) Clipboard() fyne.Clipboard {
-	//if w.clipboard == nil {
-	//	w.clipboard = &mobileClipboard{window: w.viewport}
-	//}
-	// TODO add clipboard support
+	if w.clipboard == nil {
+		w.clipboard = &mobileClipboard{}
+	}
 	return w.clipboard
 }