Fix combobox usage

src/AutoPilotPlugins/PX4/AirframeComponent.qml

@@ -229,6 +229,7 @@ Your vehicle will also be restarted in order to complete the process.")
                                 anchors.left:       parent.left
                                 anchors.right:      parent.right
                                 model:              modelData.airframes
+                                textRole:           "text"
 
                                 Component.onCompleted: {
                                     if (airframeCheckBox.checked) {

src/QmlControls/QGCComboBox.qml

@@ -37,7 +37,15 @@ ComboBox {
             width:                      control.width
 
             contentItem: Text {
-                text:                   textRole ? modelData[textRole] : modelData
+                text: {
+                    if(modelData) {
+                        return textRole ? modelData[textRole] : modelData
+                    }
+                    else {
+                        console.warn("ComboBox: no model data")
+                        return ""
+                    }
+                }
                 color:                  control.currentIndex === index ? qgcPal.buttonHighlightText : qgcPal.buttonText
                 verticalAlignment:      Text.AlignVCenter
             }