ProfileDownloadFragment: Do not crash for USB channels

app-common/src/main/java/im/angry/openeuicc/ui/ProfileDownloadFragment.kt

@@ -151,7 +151,7 @@ class ProfileDownloadFragment : BaseMaterialDialogFragment(),
         super.onStart()
         profileDownloadIMEI.editText!!.text = Editable.Factory.getInstance().newEditable(
             try {
-                telephonyManager.getImei(channel.logicalSlotId)
+                telephonyManager.getImei(channel.logicalSlotId) ?: ""
             } catch (e: Exception) {
                 ""
             }