GuiDocument::onBufferViewChanged(): exit early if dialog is not open

This should fix the bug reported at
https://marc.info/?l=lyx-users&m=171957953801548&w=2

(cherry picked from commit 574db22025)

ANNOUNCE

@@ -97,6 +97,8 @@ What's new
 
 - Fix crash when deleting rows or columns from table.
 
+- Do not ask about unapplied changes if dialog has been closed.
+
 - Fix wrong position of conversion windows of the input method (bugs 11723,
   13054).
 

src/frontends/qt/GuiDocument.cpp

@@ -1869,6 +1869,10 @@ void GuiDocument::onClosing(int const id)
 
 void GuiDocument::onBufferViewChanged()
 {
+	if (!isVisibleView())
+		// dialog not open, nothing to do
+		return;
+
 	if (switchback_) {
 		// We are just switching back. Nothing to do.
 		switchback_ = false;
@@ -1903,7 +1907,7 @@ void GuiDocument::onBufferViewChanged()
 		}
 	}
 
-	if (isVisibleView())
+	// reset params if we haven't bailed out above
 	initialiseParams("");
 }