catch NPE (#655)

app/src/main/java/com/m2049r/xmrwallet/widget/ExchangeEditText.java

@@ -68,15 +68,19 @@ public class ExchangeEditText extends LinearLayout {
         }
         boolean ok = true;
         String nativeAmount = getNativeAmount();
-        try {
-            double amount = Double.parseDouble(nativeAmount);
-            if ((amount < min) || (amount > max)) {
+        if (nativeAmount == null) {
+            ok = false;
+        } else {
+            try {
+                double amount = Double.parseDouble(nativeAmount);
+                if ((amount < min) || (amount > max)) {
+                    ok = false;
+                }
+            } catch (NumberFormatException ex) {
+                // this cannot be
+                Timber.e(ex.getLocalizedMessage());
                 ok = false;
             }
-        } catch (NumberFormatException ex) {
-            // this cannot be
-            Timber.e(ex.getLocalizedMessage());
-            ok = false;
         }
         if (!ok) {
             shakeAmountField();