readme: Add note about MacOS dmgbuild tool

README.md

@@ -27,10 +27,11 @@ To install python and cygwin dependencies and build everything use
 `deploy\windows-build.bat` batch script.
 
 ### MacOS
-Currently MacOS builds require *Python 3.6* and Qt SDK installed (just the "Qt >
+Currently MacOS builds require *Python 3.6*, `dmgbuild` tool (`pip3 install
+dmgbuild`) and Qt SDK installed (just the "Qt >
 5... > macOS" part in installer) with following added to $PATH:
 
-    export PATH="$HOME/Qt/5.11.1/clang_64/bin:$PATH"
+    export PATH="$HOME/Qt/5.11.1/clang_64/bin:/Library/Frameworks/Python.framework/Versions/3.6/bin:$PATH"
 
 Then just install dependencies and build everything using:
 

luftdaten-tool.py

@@ -52,6 +52,7 @@ class MainWindow(QtWidgets.QMainWindow, mainwindow.Ui_MainWindow):
 
         self.statusbar.showMessage(self.tr("Loading firmware list..."))
 
+        self.versionBox.clear()
         self.firmware_list = FirmwareListThread()
         self.firmware_list.listLoaded.connect(self.populate_versions)
         self.firmware_list.error.connect(self.on_work_error)
@@ -200,8 +201,12 @@ class MainWindow(QtWidgets.QMainWindow, mainwindow.Ui_MainWindow):
             self.statusbar.showMessage(self.tr("No version selected."))
             return
 
-        orig_version = self.versionBox.model().item(
-            self.versionBox.currentIndex()).text()
+        sel =  self.versionBox.model().item(
+            self.versionBox.currentIndex())
+        if sel:
+            orig_version = sel.text()
+        else:
+            orig_version = ''
 
         if version == orig_version:
             # Editable combobox has been unchanged