Add null check when getting fragment host wire provider

If the host is missing or does not build an NPE occurred (issue #1819)

Author: Kummallinen

ui/org.eclipse.pde.core/src/org/eclipse/pde/internal/core/DependencyManager.java

@@ -27,6 +27,7 @@
 
 import org.eclipse.core.resources.IResource;
 import org.eclipse.core.runtime.CoreException;
+import org.eclipse.core.runtime.ILog;
 import org.eclipse.osgi.service.resolver.BundleDescription;
 import org.eclipse.osgi.service.resolver.State;
 import org.eclipse.pde.core.plugin.IPluginModelBase;
@@ -206,7 +207,12 @@ public static Set<BundleDescription> findRequirementsClosure(Collection<BundleDe
 					// to ensure it's added again below. In the subsequent
 					// processing this fragment's requirements will then also be
 					// considered (before it was discarded).
-					closure.remove(hostWire.getProvider());
+					if(hostWire.getProviderWiring() != null) {
+						closure.remove(hostWire.getProvider());
+					} else {
+						ILog.of(DependencyManager.class).error(
+								String.format("Error resolving host of %s", wiring.getRevision().getSymbolicName())); //$NON-NLS-1$
+					}
 				}
 			}