fix a crash using middle click scroll on chat window (#1870)

* fix middle click scroll crash

* Update CHANGELOG.md

* get the float dpi, not int

src/widgets/helper/ChannelView.cpp

@@ -1969,8 +1969,7 @@ void ChannelView::disableScrolling()
 
 void ChannelView::scrollUpdateRequested()
 {
-    const qreal dpi =
-        QGuiApplication::screenAt(this->pos())->devicePixelRatio();
+    const qreal dpi = this->devicePixelRatioF();
     const qreal delta = dpi * (this->currentMousePosition_.y() -
                                this->lastMiddlePressPosition_.y());
     const int cursorHeight = this->cursors_.neutral.pixmap().height();