validate position list before touching hardware; both engines

src/main/java/org/micromanager/lightsheetmanager/model/acquisitions/AcquisitionEngineDispim.java

@@ -70,6 +70,16 @@ boolean setup() {
             return false; // early exit => save location unusable
         }
 
+        // make sure that there are positions in the PositionList; a pure read, so it belongs
+        // above the hardware changes for the same reason as the save location check
+        if (acqSettings_.isUsingMultiplePositions()) {
+            final int numPositions = studio_.positions().getPositionList().getNumberOfPositions();
+            if (numPositions == 0) {
+                studio_.logs().showError("XY positions expected but the position list is empty");
+                return false;
+            }
+        }
+
         return true;
     }
 
@@ -401,9 +411,6 @@ public void close() {
 
         // Loop 1: XY positions
         PositionList pl = MMStudio.getInstance().positions().getPositionList();
-        if (acqSettings_.isUsingMultiplePositions() && (pl.getNumberOfPositions() == 0)) {
-            throw new RuntimeException("XY positions expected but position list is empty");
-        }
 
         String[] cameraNames;
         if (demoMode) {

src/main/java/org/micromanager/lightsheetmanager/model/acquisitions/AcquisitionEngineScape.java

@@ -85,6 +85,16 @@ boolean setup() {
             return false; // early exit => save location unusable
         }
 
+        // make sure that there are positions in the PositionList; a pure read, so it belongs
+        // above the hardware changes below for the same reason as the save location check
+        if (acqSettings_.isUsingMultiplePositions()) {
+            final int numPositions = studio_.positions().getPositionList().getNumberOfPositions();
+            if (numPositions == 0) {
+                studio_.logs().showError("XY positions expected but the position list is empty");
+                return false;
+            }
+        }
+
 //        // check pixel size
 //        if (core_.getPixelSizeUm() < 1e-6) {
 //            studio_.logs().showError(
@@ -120,14 +130,6 @@ boolean setup() {
             return false;
         }
 
-        // make sure that there are positions in the PositionList
-        if (acqSettings_.isUsingMultiplePositions()) {
-            final int numPositions = studio_.positions().getPositionList().getNumberOfPositions();
-            if (numPositions == 0) {
-                studio_.logs().showError(("XY positions expected but the position list is empty"));
-                return false;
-            }
-        }
 
         return true;
     }